Research Papers

A Validated Numerical-Experimental Design Methodology for a Movable Supersonic Ejector Compressor for Waste-Heat Recovery

[+] Author and Article Information
Sajad Alimohammadi

Department of Mechanical and
Manufacturing Engineering,
Trinity College,
Dublin 2, Ireland
e-mail: alimohas@tcd.ie

Tim Persoons, Darina B. Murray

Department of Mechanical and
Manufacturing Engineering,
Trinity College,
Dublin 2, Ireland

Mohamadreza S. Tehrani, Bijan Farhanieh

School of Mechanical Engineering,
Sharif University of Technology,
P.O. Box 11155-9567,
Tehran, Iran

Juergen Koehler

Institute for Thermodynamics,
Technical University of Braunschweig,
Braunschweig 38106, Germany

1Corresponding author.

Manuscript received February 20, 2013; final manuscript received July 10, 2013; published online October 25, 2013. Assoc. Editor: Hongbin Ma.

J. Thermal Sci. Eng. Appl 6(2), 021001 (Oct 25, 2013) (10 pages) Paper No: TSEA-13-1038; doi: 10.1115/1.4025090 History: Received February 20, 2013; Revised July 10, 2013

The aim of this paper is to develop the technical knowledge, especially the optimum geometries, for the design and manufacturing of a supersonic gas–gas ejector for a waste-heat driven vehicle cooling system. Although several studies have been performed to investigate the effects of geometrical configurations of gas–gas ejectors, a progressive design methodology of an ejector compressor for application to a vehicle cooling system has not yet been described. First, an analytical model for calculation of the ejector optimum geometry for a wide range of operating conditions is developed, using R134a as the working fluid with a rated cooling capacity of 2.5 kW. The maximum values of entrainment ratio (ω) have been estimated by correlation of the main parameters in a nondimensional form. The optimum values of nozzle throat diameter (dnt) and mixing chamber diameter (dmc) thus obtained are used as a starting point for the computational fluid dynamics (CFD) optimization covering a wide range of geometrical configurations. To assess the effect of various dimensional quantities, an optimization technique has been proposed for calculation of the most efficient geometry of the target ejector for manufacturing. Using a vehicle cooling system as a test case, the final optimized dimensions are reported and discussed. An experimental validation confirms the CFD results and the ejector performance with a normalized deviation of 5% between observed and simulated results, demonstrating that the methodology is a valid ejector design tool for a wide range of applications.

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Grahic Jump Location
Fig. 1

Schematic diagram of (a) a vehicle cooling system refrigeration cycle and (b) the ejector

Grahic Jump Location
Fig. 2

Geometry of solution domain and genetrated grid for CFD simulation of the ejector

Grahic Jump Location
Fig. 3

Mach contours of flow in the ejector for an upstream pressure of 27.5 bars, and two downstream pressures (a) 4.8 bars (showing positive outlet flow) and (b) 5.2 bars (showing negative outlet flow)

Grahic Jump Location
Fig. 4

Variation of the entrainment ratio with area ratio for three different (a) evaporator temperatures (at Tg = 85 °C, Tc = 40 °C); (b) condenser temperatures (at Tg = 85 °C, Te = 5 °C); and (c) generator temperatures (at Tc = 40 °C, Te = 5 °C)

Grahic Jump Location
Fig. 5

Effect on entrainment ratio ω of (a) compression ratio Ψ and driving pressure ratio ξ (for area ratio φ = 4.2) and (b) compression ratio Ψ and area ratio φ, for the geometry chosen in Sec. 3.1.1 and for saturated generator temperature = 85 °C.

Grahic Jump Location
Fig. 6

(a) Mach contour of flow inside the ejector, (b) pressure distribution of motive and entrained flow along the ejector length

Grahic Jump Location
Fig. 7

Variation of (a) entrainment ratio with downstream pressure (kPa) and (b) static pressure along the ejector center line (PM = 6 bars), for different nozzle throat diameters (Pp = 25.26 bars and Ps = 3.496 bars; dne = 5 mm and dmc = 7 mm)

Grahic Jump Location
Fig. 8

Variation of (a) entrainment ratio with downstream pressure (kPa) and (b) static pressure along the ejector center line (PM = 6 bars), for different nozzle exit diameters (Pp = 25.26 bars and Ps = 3.496 bars; dnt = 3 mm and dmc = 7 mm)

Grahic Jump Location
Fig. 9

Effect of (a) mixing chamber diameter (for lmc = 120 mm) and (b) length (for dmc = 7 mm) on the variation of entrainment ratio with downstream pressure (kPa) (Pp = 25.26 bars and Ps = 3.496 bars; dnt = 3 mm and dne = 5 mm). (c) Velocity profile distribution at different cross sections inside the ejector (lmc = 120 mm)

Grahic Jump Location
Fig. 10

Effect of NEP (mm) on (a) entrainment ratio, (b) static pressure along the ejector center line (for Tp = 85 °C), and (c) Mach contours (dnt = 3 mm, dne = 5 mm, and dmc = 7 mm)

Grahic Jump Location
Fig. 11

Variation of static pressure along the ejector center line for different mixing chamber lengths (Pp = 25.26 bars and Ps = 3.496 bars; dnt = 3 mm, dne = 5 mm, and dmc = 7 mm)




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