Optimum design of heat exchanger for environmental control system of an aircraft using entropy generation minimization (EGM) technique

In this paper, the geometrical parameters of two heat exchangers in a typical commercial aircraft’s ECS system are designed using the Entropy Generation Minimization (EGM) design technique. The irreversibilities of all the thermodynamic devices in the system are incorporated in the numerical analysis to minimize the exergy destruction of the system. The ECS analysed was based on a bootstrap air cycle with two cooling streams; ram air and air bled from engine fan. The paper proposes optimum air conditions at each device in the system. Trends of varying numerous system parameters against entropy generation number are also investigated to provide the design direction. INTRODUCTION Application of the cooling devices in commercial aircraft is subjected to space and weight constraints. The second largest destruction of exergy on an aircraft occurs in the Environmental Control System (ECS) [1]. Due to economic constraints in the industry, current methods employed for thermodynamic optimization of power and cooling devices in aircrafts are inadequate [2]. In account of the aforementioned reasons the EGM technique was used to show that the geometric configuration of compact cross-flow heat exchangers can be deduced by optimizing the global performance of the Environmental Control System (ECS). The study was based on the bootstrap air cycle ECS consisting of engine components (diffuser, fan and compressor), precooler heat exchanger and an air cycle machine (compressor, ACM heat exchanger and turbine) A comparable air cycle was used by Bejan A et al, and Pérez-Grande et al. in their application of the EGM design tool on aircraft ECS system [3, 4, 1]. However, in most literature the heat exchangers were optimized in isolation from the ECS system. The study optimized the two heat exchangers through integration of all the thermodynamic devices in the system to minimize the exergy destruction of the entire ECS. ECS focused on employed two cooling streams, the usual ram air and one bled off from the fans to be used in the ACM and the pre-cooler heat exchangers respectively. With fixed external parameters, the entropy generation number of the ECS was written as a function of the free geometrical parameters of the two heat exchangers. This was achieved by using the laws of thermodynamics, heat transfer and fluid mechanics principals. NOMENCLATURE CC∗ Capacity ratio TT Temperature (°C) bb Constant, RR/ccpp AA Total heat transfer area(mm2) KKcc Contraction coefficient nnff Total number of fins AAffff Core frontal area(mm2) VVpp Volume side plates (mm3) VV Core volume(mm3) Subscripts ss Entropy (JJ KKKK · KK ⁄ ) aa Ambient condition NNss Entropy generation number cc Compressor ?̇?Sgggggg Entropy generation rate (WW/KK) ee Conditioning air KKgg Expansion coefficient dd Diffuser NNff Fin density(1/mm) ffaann Fan ll Fin length h Hot fluid stream tt Fin thickness(mmmm) ii Inlet fluid stream ff Flow friction factor mmiinn Minimum RRff Fouling factor nn Nozzle AAoo Free flow area(mm2) oooott Optimum h Heat transfer coefficient(WW KK · mm2 ⁄ ) oo Outlet fluid stream DDh Hydraulic diameter(mm) ww Parting wall RRaaaaff Ideal Gas constant (JJ/kkKKKK) rr Ram air h′ Internal fin height(mm) tt Turbine LLxx,yy,zz Lengths(mm) Greek symbols ?̇?m Mass flow rate(kkKK/ss) ρρ Density (kkKK/mm3) GG Mass velocity(kkKK ss · mm2 ⁄ ) ττ Dimensionless temperature NNpp Number of passages μμh,cc Dynamic viscosity(PPaa · ss) NNNN Nusselt number ηηff Fin efficiency UU Overall heat transfer coefficient(WW KK · mm2 ⁄ ) γγ Fin spacing to height ratio PPrr Prandtl number εε Heat exchanger effectiveness PP Pressure(PPaa) ηη Isentropic efficiency PP Pressure(kkPPaa) ηηoo Overall surface efficiency ?̇?Q Rate of heat transfer (WW) σσ Porosity RRee Reynolds number μμ1,2 Ratio of capacity rates ccpp Specific heat capacity (JJ/kkKKKK) ββ Surface area density(mm2/mm3) SStt Stanton number κκ Thermal conductivity (WW mm · KK ⁄ ) Optimum flow path length was used to set the entropy generation number function of the two heat exchangers was along a minimum path before being integrated into the whole system. Ten programmed numerical methods were used to obtain the minimum entropy generation number of the ECS. Optimization results include the geometrical parameters of the heat exchangers, optimum system air condition configuration and entropy generation number of all the devices in the system. Results obtained from similar studies were used as a base of optimization comparison. Trends were also developed to show the effect of varying the core dimensions of the heat exchangers on the entropy generation number of the entire system, providing some design direction. 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics