Research Papers

J. Thermal Sci. Eng. Appl. 2015;7(4):041001-041001-15. doi:10.1115/1.4030478.

Processing of glass is indeed challenging owing to its chemical passivity; it is prone to cracking while processing through mechanical and thermal modes without appropriate strategies. Near-field microwave drilling is a thermal-ablation based material removal technique of generating high heat flux in the targeted area. Glasses tend to fail quite frequently during this processing owing to thermal stresses (shock). It was therefore important to develop suitable strategies to minimize cracking during this potentially pragmatic process for microdrilling. Accordingly, in the present work, an attempt was made to change the medium of the interface at the target drilling zone through application of seven different surface precursors to influence the local heat-flow characteristics. The cracking behavior of the soda lime glass during microwave drilling in a customized applicator under controlled power input (90–900 W) at 2.45 GHz was investigated. The heat was generated inside the applicator by creating a plasma sphere in the drilling zone through a metallic concentrator. The thermal shock on the glass specimen was found reduced by the combination of a good dielectric precursor and microwave concentration for hotspot formation, which in turn, reduces the cracking/crazing tendency. Trials were carried out while drilling holes on 1.2 mm thick glass plates at various duty cycles (DCs) to study the crack intensity and pattern. The near-field microwave drilling condition was also simulated to obtain the contours of the induced stresses. The results so obtained were compared with the cracking signatures of the experimental outputs; a good correlation could be obtained. It was found that both solid and liquid fluxes as precursor could be effective to control cracking during microwave drilling.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041002-041002-9. doi:10.1115/1.4030635.

This study presents an experimental investigation of the characteristics of convective heat transfer in horizontal shell and coil heat exchangers in addition to the friction factor for fully developed flow through their helically coiled tube (HCT). Five heat exchangers of counterflow configuration were constructed with different HCT-curvature ratios (δ) and tested at different mass flow rates and inlet temperatures of γ-Al2O3/water nanofluid in the HCT. The tests were performed for γ-Al2O3 with average size of 40 nm and particles volume concentration (ϕ) from 0% to 2% for 0.0392δ0.1194. Totally, 750 test runs were performed from which the HCT-average Nusselt number (Nu¯t) and fanning friction factor (fc) were calculated. Results illustrated that Nu¯t and fc of nanofluids are higher than those of the pure water at same flow condition, and this increase goes up with the increase in ϕ. When ϕ increases from 0% to 2%, the average increase in Nu¯t is of 59.4–81% at lower and higher HCT-Reynolds number, respectively, and the average increase in fc is of 25.7% and 27.4% at lower and higher HCT-Reynolds number, respectively, when ϕ increases from 0% to 2% for δ=0.1194. In addition, results showed that Nu¯t and fc increase by increasing coil curvature ratio. When δ increases from 0.0392 to 0.1194 for ϕ=2%, the average increase in Nu¯t is of 130.2% and 87.2% at lower and higher HCT-Reynolds number, respectively, and a significant increase of 18.2–7.5% is obtained in the HCT-fanning friction factor at lower and higher HCT-Reynolds number, respectively. Correlations for Nu¯t and fc as a function of the investigated parameters are obtained.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041003-041003-10. doi:10.1115/1.4030636.

Heat and mass transfer in a planar anode-supported solid oxide fuel cell (SOFC) module, with bipolar-plate interconnect flow channels of different shapes are computationally simulated. The electrochemistry is modeled by uniform supply of volatile species (moist hydrogen) and oxidant (air) to the electrolyte surface with constant reaction rate via interconnect channels of rectangular, trapezoidal, and triangular cross sections. The governing three-dimensional equations for fluid mass, momentum, energy, and species transport, along with those for electrochemical kinetics, where the homogeneous porous-layer flow is in thermal equilibrium with the solid matrix, are coupled with the electrochemical reaction rate to properly account for the heat and mass transfer across flow-ducts and electrode-interfaces. The results highlight effects of interconnect duct shapes on lateral temperature and species distributions as well as the attendant frictional losses and heat transfer coefficients. It is seen that a relatively shallow rectangular duct offers better heat and mass transfer performance to affect improved thermal management of a planar SOFC.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041004-041004-7. doi:10.1115/1.4030637.

Solar thermoelectric generators (STEGs) convert solar energy to electricity. The solar energy is first used to heat an absorber plate that serves as the high temperature reservoir. Power is generated by connecting the hot reservoir and cold (ambient) reservoirs with a pair of p- and n-doped thermoelectric legs. Experimental studies have shown that the efficiency of a STEG can reach values of about 5% if the entire setup is placed in near-vacuum conditions. However, under atmospheric conditions, the efficiency decreases by more than an order of magnitude, presumably due to heat loss from the absorber plate by natural convection. A coupled fluid–thermal–electric three-dimensional computational model of a STEG is developed with the objective of understanding the various loss mechanisms that contribute to its poor efficiency. The governing equations of mass, momentum, energy, and electric current, with the inclusion of thermoelectric effects, are solved on a mesh with 60,900 cells, and the power generated by the device is predicted. The computational model predicts a temperature difference (ΔT) of 16.5 K, as opposed to the experimentally measured value of 15 K. This corresponds to a peak power of 0.031 W as opposed to the experimentally measured peak power of 0.021 W. When only radiative losses are considered (i.e., perfect vacuum), the ΔT increases drastically to 131.1 K, resulting in peak power of 1.43 W. The predicted peak efficiency of the device was found to be 0.088% as opposed to the measured value of 0.058%.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041005-041005-7. doi:10.1115/1.4030638.

A methodology is developed for the design of an air-cooled 55-kW-rated inverter heat sink. The design constraints are that the power density (PD) must meet or exceed the values associated with liquid-cooled systems of the same power rating, and that the maximum surface temperatures be less than 200 °C. To keep the pressure drop low relative to turbulent flow designs, a laminar flow regime is chosen. A preliminary design that satisfies the PD constraint exactly, and the thermal requirements approximately, is determined. To ensure that the thermal requirements are met by the design configuration, a thermal-fluid analysis based on a three-dimensional conjugate heat transfer model is conducted. Overall, energy balance errors (OEBEs) as high as 15% were encountered in the numerical models. These errors are reduced by taking advantage of the symmetry between fins using a typical unit cell model. A new simplified approach for the simulations was identified which involved modeling fins as highly conductive layers instead of solid domains. This further reduced the OEBEs to less than 0.004%. The design factors considered in this study include effective cooling surface area, fin thickness, fin spacing, and fin height. The results show that the maximum surface temperatures can be kept below 200 °C for safe operation of SiC devices in the inverter module while increasing the PD.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041006-041006-13. doi:10.1115/1.4030639.

The vortex tube (VT) air separator is an invaluable tool which has the ability to separate a high-pressure fluid into the cold and hot fluid streams. The hot tube is a main part of the air separator VT which the energy separation procedure happens along this part. This research has been done to analyze the effect of the convergent angle and cold orifice diameter on the thermal efficiency of a convergent vortex tube (CVT). The CVT is linked to an air pipeline with the fixed pressure of 6.5 bar. The convergent hot tube angle is varied over the range of 1 deg to 9 deg. The consideration of the main angle effect denotes that the highest thermal ability could be achieved at β = 5 deg. The laboratory setup results show this subject that the optimization of the hot tube convergent angle elevates the cooling and heating effectiveness around 32.03% and 26.21%, respectively. Experiments denoted that both cooling capability and heating effectiveness reach the highest magnitudes when the DCold is around 9 mm. After these two stages, the optimized CVT was capable of decreasing and rising air temperatures at the cold and the hot sides up to 9.05 K (42.89%) and 10.48 K (44.74%), respectively. A computational fluid dynamics (CFD) model was employed to predict the performance of the air flow inside the CVT. The numerical investigation was done by full 3D steady-state CFD-simulation using fluent6.3.26. The results show that the agreement between computation predictions and laboratory measurements is fairly good.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041007-041007-8. doi:10.1115/1.4030697.

A hydrogen-producing solar reactor was experimentally tested to study the cyclone flow dynamics of the gas–particle two-phase phenomenon. Two-dimensional particle image velocimetry (PIV) was used to observe the flow and to quantify the vortex formation inside the solar reactor. The vortex flow structure in the reactor was reconstructed by capturing images from orientations perpendicular and parallel to the geometrical axis of the reactor, respectively. The experimental results showed that the tangential components of the fluid velocity formed a Rankine-vortex profile. The free vortex portions of the Rankine profile were synchronized and independent of the axial position. The axial components showed a vortex funnel of higher speed fluid supplied by a reversing secondary flow. According to the inlet channel design, the geometry dominates the flow dynamics. A stable processing vortex line was observed. As the vortex flow evolves toward the exit, the vortex funnel expands radially with decreasing tangential velocity magnitude peak as a result of the vortex stretching. An optimal residence time of the flow was found by changing the cyclone flow inlet conditions. The swirl number versus the main flow rate change was obtained. Upon completion of the experimental studies, a thorough numerical analysis was conducted to model the flow dynamics inside the solar reactor and to verify the results by comparison to the experimental results. Three turbulence models including the standard k–ϵ, k–ϵ renormalization groups (RNG), and Reynolds stress transport models were used. Computational fluid dynamics (CFD) simulations were coupled with heat transfer analysis via discrete ordinate (DO) model. Particle tracing in Lagrange frame was applied to simulate the particle trajectory. A comparison between the turbulence modeling results for the room temperature and high temperature cases, as well as the experimental results for room temperature cases is presented.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041008-041008-9. doi:10.1115/1.4030640.

Simulating the real-time thermal behavior of rooms subject to air conditioning (AC) and refrigeration is a key to cooling load calculations. A well-established resistance–capacitance (RC) model is employed that utilizes a representative network of electric resistors and capacitors to simulate the thermal behavior of such systems. A freezer room of a restaurant is studied during its operation, and temperature measurements are used for model validation. Parametric study is performed on different properties of the system. It is shown that a reduction of 20% in the walls thermal resistivity can increase the energy consumption rate by 15%. The effect of set points on the number of compressor starts/stops is also studied, and it is shown that narrow set points can result in a steady temperature pattern in exchange for a high number of compressor starts/stops per hour. The proposed technique provides an effective tool for facilitating the thermal modeling of air conditioned and refrigerated rooms. Using this approach, engineering calculations of cooling load can be performed with outstanding simplicity and accuracy.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041009-041009-9. doi:10.1115/1.4030792.

The experimental friction factor and Nusselt number data for laminar flow of viscous oil through a circular duct having integral transverse rib roughness and fitted with twisted tapes with oblique teeth have been presented. Predictive friction factor and Nusselt number correlations have also been presented. The thermohydraulic performance has been evaluated. The major findings of this experimental investigation are that the twisted tapes with oblique teeth in combination with integral transverse rib roughness perform significantly better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain value of fin parameter.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041010-041010-10. doi:10.1115/1.4030814.

Lack of proper ventilation of exhaust fumes from gas fired stoves in residential kitchens is a major health concern for some populations. It could even cause destruction of property, and reduce quality of life and lifespan. In this study, a typical kitchen having a standard dimension of 2.13 m × 2.43 m × 3.05 m was modeled with single open door exit. Two heat sources were used for modeling the kitchen that resembles the double burner gas stove of an urban residential kitchen in developing countries. Steady-state simulations were performed using a three-dimensional computational fluid dynamics (cfd) code with appropriate boundary conditions. The present numerical method was validated by comparing with the experimental data reported by Posner et al. (2003, “Measurement and Prediction of Indoor Air Flow in a Model Room,” J. Energy Build., 35(5), pp. 515–526). The comparison showed very reasonable agreement. A grid independence test was also performed to determine the optimum grid resolution reflecting the accuracy of the numerical solution. The results are presented for carbon dioxide (CO2) gas emission from the stove exhaust and dispersion within the kitchen space. A comparative analysis between the ventilation (natural and forced) and no ventilation conditions is also reported in this study. The location of the breathing zone was at a height of 73 cm and at a distance of 33 cm from the center of the two burners. Very high concentration (above 5000 ppm) of CO2 gas was observed at the plane passing the breathing zone. Exposure to this environment for longer time may cause serious health damage of the occupants (http://www.dhs.wisconsin.gov/eh/chemfs/fs/carbondioxide.htm). As per the Wisconsin Department of Health Services of USA, over 5000 ppm exposures to CO2 lead to serious oxygen deficit resulting in permanent brain damage, coma, and even death.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041011-041011-17. doi:10.1115/1.4030882.

Conjugate heat transfer in a two-dimensional, steady, incompressible, confined, turbulent slot jet impinging normally on a flat plate of finite thickness is one of the important problems as it mimics closely with industrial applications. The standard high Reynolds number two-equation k–ε eddy viscosity model has been used as the turbulence model. The turbulence intensity and the Reynolds number considered at the inlet are 2% and 15,000, respectively. The bottom face of the impingement plate is maintained at a constant temperature higher than the jet exit temperature and subjected with constant heat flux for the two cases considered in the study. The confinement plate is considered to be adiabatic. A parametric study has been done by analyzing the effect of nozzle-to-plate distance (4–8), Prandtl number of the fluid (0.1–100), thermal conductivity ratio of solid to fluid (1–1000), and impingement plate thickness (1–10) on distribution of solid–fluid interface temperature, bottom surface temperature (for constant heat flux case), local Nusselt number, and local heat flux. Effort has been given to relate the heat transfer behavior with the flow field. The crossover of distribution of local Nusselt number and local heat flux in a specified region when plotted for different nozzle-to-plate distances has been discussed. It is found that the Nusselt number distribution for different thermal conductivity ratios of solid-to-fluid and impingement plate thicknesses superimposed with each other indicating that the Nusselt number as a fluid flow property remains independent of solid plate properties.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041012-041012-9. doi:10.1115/1.4031018.

A significant step in the design of heating, ventilating, air conditioning, and refrigeration (HVAC-R) systems is to calculate room thermal loads. The heating/cooling loads encountered by the room often vary dynamically while the common practice in HVAC-R engineering is to calculate the loads for peak conditions and then select the refrigeration system accordingly. In this study, a self-adjusting method is proposed for real-time calculation of thermal loads. The method is based on the heat balance method (HBM) and a data-driven approach is followed. Live temperature measurements and a gradient descent optimization technique are incorporated in the model to adjust the calculations for higher accuracy. Using experimental results, it is shown that the proposed method can estimate the thermal loads with higher accuracy compared to using sheer physical properties of the room in the heat balance calculations, as is often done in design processes. Using the adjusted real-time load estimations in new and existing applications, the system performance can be optimized to provide thermal comfort while consuming less overall energy.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041013-041013-9. doi:10.1115/1.4031082.

Lithium bromide solution is used as a desiccant in air dehumidification systems. Liquid desiccant is a solution that facilitates the removal of humidity directly from the air. In this work, effectiveness of a LiBr based air dehumidifier was studied by correlating the vapor–liquid equilibrium data with a proposed thermodynamic model. For this, the nonelectrolyte Wilson nonrandom factor (N-Wilson-NRF) model and the Pitzer–Debye–Huckel formula were used to represent the contribution of the short and the long range ion–ion interactions. In particular, the proposed model assumed that the electrolyte solution is treated as a mixture of undissociated ion pairs and solvent molecules. The proposed equation of this study is valid for the temperature range of 20–35 °C and concentration range of 0.40–0.60 kg/kg. This relation was employed to estimate the equivalent humidity ratio, and then, the humidity ratio from the previous step was used to calculate the effectiveness of a LiBr based dehumidifier. The response surface methodology (RSM) was applied for the design and analysis of the dehumidification experiments. A quadratic model was implemented to predict the dehumidification effectiveness. This model studies the implications of four primary variables on the effectiveness of a dehumidification process. The optimal values to achieve the maximum effectiveness were found to be 32.5 °C for the air temperature, 0.0210 kg/kg for the air humidity ratio, 2.17 for the mass flow rate ratio, and finally, 0.50 kg/kg for the desiccant concentration. These values gave the dehumidification effectiveness of 0.544. The result of the model was in good agreement with the experimental value 0.542, thus verifying the accuracy of the proposed model.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041014-041014-9. doi:10.1115/1.4031132.

Heat rejection for space suit thermal control is typically achieved by sublimating water ice to vacuum. Converting the majority of a space suit's surface area into a radiator may offer an alternative means of heat rejection, thus reducing the undesirable loss of water mass to space. In this work, variable infrared (IR) emissivity electrochromic materials are considered and analyzed as a mechanism to actively modulate radiative heat rejection in the proposed full suit radiator architecture. A simplified suit geometry and lunar pole thermal environment is used to provide a first-order estimate of electrochromic performance requirements, including number of individually controllable pixels and the emissivity variation that they must be able to achieve to enable this application. In addition to several implementation considerations, two fundamental integration architecture options are presented—constant temperature and constant heat flux. With constant temperature integration, up to 48 individual pixels with an achievable emissivity range of 0.169–0.495 could be used to reject a metabolic load range of 100 W–500 W. Alternatively, with constant heat flux integration, approximately 400 pixels with an achievable emissivity range of 0.122–0.967 are required to reject the same load range in an identical external environment. Overall, the use of variable emissivity electrochromics in this capacity is shown to offer a potentially feasible solution to approach zero consumable loss thermal control in space suits.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041015-041015-9. doi:10.1115/1.4031220.

To assess the potential of thermal energy storage systems using phase change materials (PCMs), numerical simulations rely on an enthalpy–temperature curve (or equivalent specific heat curve) to model the PCM thermal storage behavior. The so-called “T-history method” can be used to obtain an enthalpy–temperature curve (H versus T) through conventional laboratory equipment and a simple experimental procedure. Different data processing variants of the T-history method have been proposed yet no systematic comparison between these versions exists in the literature nor is there a consensus as to which should be used to obtain reliable enthalpy–temperature curves. In this paper, an inorganic salt hydrate is tested in both heating and cooling. Four different data processing variants of the T-history method are used to characterize the PCM and produce enthalpy–temperature curves for this original experimental data set. Differences in the results produced by the different methods are discussed, the issues encountered are indicated, and possible approaches to overcome these problems are provided. A specific variant is recommended when using the T-history method to determine enthalpy–temperature curves. For PCMs that exhibit subcooling, an alternative interpretation using an absolute temperature interval is described so that the subcooling phase is taken into account in the enthalpy–temperature curve.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041016-041016-13. doi:10.1115/1.4031221.

Laminar mixed convection in a two-dimensional shallow inclined lid-driven cavity is investigated numerically. The moving cavity lid at the top is isothermally hot and the bottom is isothermally cold while the two sidewalls are insulated. The cavity aspect ratio is taken as 10. The fluid medium consists of a mixture of pure water and copper nanoparticles with volumetric concentrations of 5% and 8%. The flow Richardson number is varied from 0.1 to 10, and the cavity inclination is varied from 0 deg to 30 deg. It is found that, at any specific nanoparticle concentration, the average Nusselt number increases mildly with cavity inclination for the forced convection dominated case (Ri = 0.1) while it increases much more rapidly with inclination for natural convection dominated case (Ri = 10). Also the average Nusselt number has significant increasing trend with increasing concentration of the nanoparticles.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041017-041017-10. doi:10.1115/1.4031222.

Heat exchangers are important facilities that are widely used in heating, ventilating, and air conditioning (HVAC) systems. For example, heat exchangers are the primary units used in the design of the heat transfer loops of cooling systems for data centers. The performance of a heat exchanger strongly influences the thermal performance of the entire cooling system. The prediction of transient phenomenon of heat exchangers is of increasing interest in many application areas. In this work, a dynamic thermal model for a cross flow heat exchanger is solved numerically in order to predict the transient response under step changes in the fluid mass flow rate and the fluid inlet temperature. Transient responses of both the primary and secondary fluid outlet temperatures are characterized under different scenarios, including fluid mass flow rate change and a combination of changes in the fluid inlet temperature and the mass flow rate. In the ε-NTU (number of transfer units) method, the minimum capacity, denoted by Cmin, is the smaller of Ch and Cc. Due to a mass flow rate change, Cmin may vary from one fluid to another fluid. The numerical procedure and transient response regarding the case of varying Cmin are investigated in detail in this study. A review and comparison of several journal articles related to the similar topic are performed. Several sets of data available in the literatures which are in error are studied and analyzed in detail.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041018-041018-10. doi:10.1115/1.4031358.

In this paper, a formulation for the rate of heat generation due to the contact of one asperity with asperities on a second surface is proposed. A statistical approach is used to obtain the heat generation rate due to one asperity and employed to develop the equation for generation of heat rate between two rough surfaces. This heat rate formulation between the two rough surfaces has been incorporated into the 2D lumped parameter model of disk pair in dry friction developed by Elhomani and Farhang (2010, “A 2D Lumped Parameter Model for Prediction of Temperature in C/C Composite Disk Pair in Dry Friction Contact,” ASME J. Therm. Sci. Eng. Appl., 2(2), p. 021001). In this paper, the disk brake is viewed as consisting of three main regions: (1) the surface contact, (2) the friction interface, and (3) the bulk. Both surfaces of the disk brake are subjected to frictional heating. This model is considered to be a necessary step for simulating the aircraft braking system that consists of a stack of multiple disks.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041019-041019-11. doi:10.1115/1.4031425.

The thermal characteristics of Bingham plastic fluid flows are analyzed in circular microchannels under uniform wall heat flux condition. The analytic approach presented here reveals that the governing parameters are Bingham number, dimensionless radius of the plug flow region, and Brinkman number. The results demonstrate that there is a strong influence of viscous dissipation on heat transfer and entropy generation for Brinkman numbers greater than a specific value. With increasing the Brinkman number and dimensionless radius of the plug flow region, entropy generation is increased, while the Nusselt number is decreased. The influence of these parameters on the entropy generation from heat transfer is strongly higher than the entropy generation from fluid friction. The average dimensionless total entropy shows that the Bingham plastic fluids generate entropy more than Newtonian fluids; also, an increase in the dimensionless radius of the plug flow region results in increasing the average dimensionless total entropy generation. By letting the dimensionless radius of the plug flow region equal to zero, the generalized expressions and results will be simplified to Newtonian fluids.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041020-041020-14. doi:10.1115/1.4031357.

An experimental study of the shroud heat transfer behavior and the effectiveness of shroud cooling are undertaken in a single-stage turbine at low rotation speeds. The shroud consists of a periodic distribution of laterally oriented cooling holes that are angled at 45 deg to the shroud surface in a repeating circumferential pattern and has five unique hole pitches in the axial direction. Measurements of the normalized Nusselt number and film cooling effectiveness are done using liquid crystal thermography. These measurements are reported for the no-coolant case and nominal blowing ratios (BRs) of 1.0, 1.5, 2.0, 2.5, and 3.0. The tests are performed at an inflow Reynolds number of 17,500 corresponding to a scaled down design rotation speed of 550 rpm, and two off-design speeds imposed by a motor: (1) a rotation speed below the design speed (400 rpm) and (2) a rotation speed above the design speed (700 rpm). The results at the design speed show that increasing the BR increases the area-averaged film cooling effectiveness, while the Nu/Nu0 in the shroud hole region decreases. As the rotor speed is changed from the design speed, the high Nu/Nu0 region migrates on the shroud surface. This migration affects the coolant coverage in the shroud hole region resulting in increased coolant coverage at below-design rotation speeds and decreased coolant coverage at above-design rotation speeds. At all rotation speeds, as the BR increases, the area-averaged film cooling effectiveness in the shroud hole region increases. Decreasing the circumferential shroud coolant hole spacing changes the lateral heat transfer profile from a periodic sinusoidal distribution for a shroud hole spacing of P/D = 10.4 to a more even distribution for a smaller shroud hole spacing (P/D = 4.8).

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041021-041021-7. doi:10.1115/1.4031539.

Advances in the design and development of communication spacecraft are associated with an increase in power consumption and heat dissipation in the spacecraft. As a consequence, advanced thermal control technologies like mechanical pumped fluid loop (MPFL) are increasingly being considered for spacecraft temperature management. These technologies generally use radiative sinks (often deployable) to reject heat. Since mass is a critical parameter in space applications, mass-optimized radiator design is paramount. This paper presents semi-analytical approach to evolve design of a mass-optimized space radiator panel for single-phase MPFL.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):041022-041022-11. doi:10.1115/1.4031540.

Highly porous ceramic fiber insulations are beginning to be considered as a replacement for firebrick insulations in high temperature, high pressure applications by the chemical process industry. However, the implementation of such materials has been impeded by a lack of experimental data and predictive models, especially at high gas pressure. The goal of this work was to develop a general, applied thermophysical model to predict effective thermal conductivity, keff, of porous ceramic fiber insulation materials and to determine the temperature, pressure, and gas conditions under which natural convection is a possible mode of heat transfer. A model was developed which calculates keff as the sum of conduction, convection, and radiation partial conductivities. The model was validated using available experimental data, including laboratory measurements made by this research effort. Overall, it was concluded that natural convection is indeed possible for the most porous insulations at pressures exceeding 10 atm. Furthermore, keff for some example insulations was determined as a function of temperature, pressure, and gas environment.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Thermal Sci. Eng. Appl. 2015;7(4):044501-044501-4. doi:10.1115/1.4031359.

The present experimental study is carried out to verify previously published heat transfer results attained using a simpler yet nascent data reduction technique for the same plate heat exchanger. A gasketed, commercially available plate heat exchanger with mixed (30/60) plate configuration was used in this study to obtain experimental heat transfer coefficient using modified Wilson plot method for data reduction. The comparison between current data and previously published results has shown excellent agreement between the two techniques hence verifying the results of the simpler method used earlier.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2015;7(4):044502-044502-6. doi:10.1115/1.4031465.

This paper presents the findings of an experimental and numerical investigation on the shock effect on heat transfer coefficient and film-cooling effectiveness. In this study, coolant was injected on the blade surface through a fan-shaped hole in a transonic cascade. The experimental results indicate that on the film-cooled suction surface of the blade, the shock from the adjacent blade impinging on the suction surface causes the film-cooling effectiveness to drop quickly by 18%, and then stay at a low level downstream of the shock. The shock also causes the local heat transfer coefficient to decrease rapidly by 25%, but then rise back up immediately after the shock. The results from the numerical study supported the film-cooling effectiveness and heat transfer coefficient trends that were observed in the experiment. A detailed analysis of the numerical results reveals that the rapid change of the film-cooling effectiveness is due to the near surface secondary flows, which push the hot mainstream air toward the injection centerline and lifts the low temperature core away from the surface. This secondary flow is a result of a spanwise pressure gradient. The drop in heat transfer coefficient is caused by a boundary layer separation bubble which results from an adverse streamwise pressure gradient at the shock position.

Commentary by Dr. Valentin Fuster


J. Thermal Sci. Eng. Appl. 2015;7(4):047001-047001-1. doi:10.1115/1.4031083.

Our recently published paper [1] presents a theoretical treatment of the degradation in performance of multilayer insulation (MLI) when there is a small amount of radiative transmission through each of the insulation layers. We present predictions of this degradation obtained by a skin depth calculation of the penetration of long wave radiation impinging on the thin aluminum layers used in this insulation. As a result, we predicted observable degradation in the performance of thinly coated MLI when used as a shield between cold objects such as shielding a 4 K object from a 77 K object.

Topics: Modeling , Insulation
Commentary by Dr. Valentin Fuster

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