Al-Qadisiyah Journal for Engineering Sciences

Abstract Air-conditioning systems (ACs) are essential in hot and humid climates to ensure acceptable ambient air quality as well as thermal comfort for buildings users. It is essential to improve refrigeration system performance without increasing the effects of global warming potential (GWP) and ozone depletion potential (ODP). The main objective of this study is to evaluate the performance of an air conditioning system that operates with a liquid suction heat exchanger (LSHX) through implementing refrigerants with zero OPD and low GWP (i.e., R134a and R1234yf). Liquid suction heat exchanger (LSHX) was added to an automobile air conditioning system (AACS).When Liquid suction heat exchanger was added to the cycle, primary results indicated that an enhancement in the cycle coefficient performance (COP) by 25.2 %and 17.3% for R134a and R1234yf respectively, and decreasing in mass flow rate of the refrigerants used (m ̇_r). Also increasing in refrigeration effect (RE) by 4.2% and 2.3% for R134a and R1234yf respectively. Presence of LSHX caused increasing in heat rejected by condenser (Qcond.) according to increasing in sub-cooling degree and decreasing in compressor work (Wcomp.), increasing in condenser exergy destruction (X. des. cond.) and thermostatic expansion device (TXV) exergy destruction, but decreasing in compressor and evaporator. Summary of previous experimental and numerical studies is presented as well.

Abstract Numerically investigated the electric double layer (EDL) Effects on the performance of the square microchannel heat exchanger (MCHE) at combined electro-osmotic and pressure-driven flow with compared pure pressure-driven with a hydraulic diameter (10 – 50) μm. We defined at any size (Dh) of microchannel heat exchanger become the impact of EDL is very slight with the studied effect of electric double layer thickness λ. The diluted water 1:1 potassium chloride (KCl) solution is used as a working fluid at an ionic concentration (10^-4, 10^-6) M, silicon microchannel at zeta potential of surface -0.2 volt. A three-dimensional (3D) Poisson-Boltzmann equations and Naiver-stoke equations with applied electric field are solved by using the finite volume scheme in this work. The results show an increase in pressure drop of the microchannel heat exchanger at combined flow electroosmotic and pressure-driven flow with a percentage of 31.09 % at an ionic concentration of 10^-4 M and 42.71 % at  10^-6 Mincrease in pumping power, especially at low ionic concentration. Slight enhancement in average heat transfer rate and effectiveness due to an increase in average temperature difference. Decrease in overall performance at combined electroosmotic and pressure-driven flow compared with pure pressure driven.

Abstract Tubular heat exchanger (THEX), that has been in numerous engineering applications, represents an apparatus that makes heat to be exchanged between two fluids having different changing in temperatures and kept separated by means of a solid wall. In order to enhance the efficiency of the THEX, the rate of heat transfer at the tube side should be improved. Inserting a twisted tape inside the heat exchanger’s tube is one of the passive techniques that has been widely used to improve the heat transfer especially in air conditioning and cooling, processes of power recovery, processes for food and dairy, and plants for chemical processing. The heat exchanger enhancement is achieved by means of using a twisted tape inserted with twisting ratios (TR) equal to 3.2, 4.4, and 5.5, independently. The influences of 2-D parameters such as Nusselt number and frictional coefficient on the THEX’s effectiveness were investigated. The aim of the study is inserting a twisted tape inside the testing pipe to produced turbulent flow and, therefore, creating large turbulence rate inside the pipe that plays an significant role in improving the transferred heat and increasing the drop in the pressure. In this work, the inserted tape has a width and length equal to 21.5 mm and 1000 mm, respectively. The inner and outer diameters of the used pipes were 23 mm and 22 mm, respectively. The tested sectional length of the THEX was equal to 2000 mm.  Reynolds number was changed from 500 to 7000. Results obtained from using twisted inserting tapes with varying TR were compared with result from plain tubes. These results were displayed in the contours show the distribution of the temperature and the trajectory of the flow trajectory by axial velocity for testing the low values of Reynolds number applicability in heat exchanger applications

Abstract The use of gas turbine is increasing day by day for producing electricity and for various industrial applications. In this work a new approach of study for the effect of different cycle arrangements on the performance of simple gas turbine has been investigated. The new cycle arrangements include: reheat with heat exchanger, reheat with water injection, heat exchanger with water injection and reheat together with heat exchanger and water injection. All these arrangements are compared to the performance of gas turbine cycle with no modifications. A Matlab code was written to calculate the combustion characteristics and major performance parameters such as net work, fuel/air ratio, specific fuel consumption, and thermal efficiency ….etc. It’s observed that using reheat in addition to heat exchanger with water injection gives higher thermal efficiency, maximum increment was 28.6% in compare with normal basic cycle and lower fuel consumption, maximum reduction was 22.2 in compare with normal basic cycle.

Abstract One dimensional analysis of heat transfer from fin is simple when only heat transfer by convection from fin surface. However, in some cases heat transfers by combined phenomena (convection and radiation), this makes the governing differential equation of heat transfer non-linear and the analytical solution becomes hard to solve. This paper uses an approximate method to solve the problem with high-quality results based on finite element technique. The method can be used for different types of fins and for different shapes. The results show suitable agreement between semi-exact solution and numerical solution

Abstract A mathematical model for the unsteady state heat and mass transfer in regenerative rotary heat exchanger is derived and solved numerically by using finite difference method. Triangular shape of the rotary matrix with silica gel (SiO2) that used as desiccant material is taken here. The heat transfer coefficient and the mass transfer coefficient that used in the present study are suitable for different types of the matrix shapes. The unsteady state time effect, inlet air temperature, Number of Transfer Unit (NTUt), heat exchanger length, inlet air temperature and the desiccant material volume on the latent, sensible and total effectiveness are discussed. It has been notes that the increasing of the heat exchanger length, inlet air temperature, desiccant material volume and the (NTUt) will increase the sensible, latent and total effectiveness. The increasing of the inlet air mass flow rate will decrease the total, latent and sensible effectiveness. The steady state time for the latent effectiveness is lower than the steady state time for the sensible and total effectiveness.

Abstract The effect of the environment (surrounding) conditions on the effectiveness of the heat exchanger for different working fluids are discussed here. The heat transfer between the heat exchanger external wall and environment are considered taken by radiation and convection. The convection heat transfer coefficient between the heat exchanger wall and the environment is taken as temperature dependence and the heat exchanger external wall is assumed a grey body. We analyzed the effect of the environment conditions on two type of heat exchanger working fluid, low viscid fluid (water) and medium viscid fluid (oil SAE-30). Because of the time is very effected factor on the heat transfer, then the unsteady state of the fluid inside the heat exchanger are assumed and analyzed in the present article. The increase of the emissivity, environment heat coefficient and environment temperature will decrease the hot fluid effectiveness and increase the entropy generation number. The fluid effectiveness is very effected by the environment specially when the working fluid has lower viscosity for same flow rate. The finite difference techniques is used to solve the unsteady state differential equations for the hot and cold fluid.