Parallel Flow Microchannel Heat Exchanger Research Articles

Study surface roughness on overall performance of parallel flow microchannel heat exchanger

The aim of this paper is to investigate numerically the surface roughness effects on the overall performance of parallel flow microchannel heat exchanger (PFMCHE) with different hydraulic diameters. Water has been used as working fluid flow velocities in one direction for hot and cold channels with square shape. Surface roughness considered in this study has been choiced as ratio with equal 0.14 of surface roughness to hydraulic diameter. The obtained results showed that, the pumping power increased with increasing effect of surface roughness and decreasing hydraulic diameter, also the thermal performance increased slightly with increasing surface roughness

Effect of variable fluid properties on the hydrodynamic and thermal characteristics of parallel flow microchannel heat exchanger

In this paper the parallel flow microchannel heat exchanger is numerically investigate and the effect of treating the properties of fluids as variables with temperature compared with constant fluid properties assumption on its performance is studied. A numerical simulation is made to solve 3D Navier stock equations in two fluids with temperature dependent properties and 3D energy equation in two fluids and solid walls taking into account the effect of entrance region and axial conduction in the walls. The properties entered to the main program by relations as a function of temperature using subroutines written to calculate the properties at the end of each iteration. The results obtained show that, treating the properties as a temperature dependent gives higher overall performance since it leads to obtain lower pressure drop and higher effectiveness compared with constant properties assumption.

Experimental and Theoretical Study on the Cooling Performance of a CO2 Mobile Air Conditioning System

CO2 (GWP = 1) is considered as a promising natural alternative refrigerant to HFC-134a in mobile air conditioning (MAC) applications. The objective of this study is to investigate the cooling performance characteristics of a CO2 MAC system. A prototype CO2 MAC system, consisting of a CO2 electrical compressor, CO2 parallel flow microchannel heat exchangers, and an electrical expansion valve, was developed and tested. Factor analysis experiments were conducted to reveal the effect of outdoor temperature on the cooling performance of this CO2 MAC system. Compared with a conventional R134a MAC system, the prototype CO2 MAC system achieved comparable cooling capacity, but had COP reductions of 26% and 10% at 27 °C and 45 °C outdoor conditions, respectively. In addition, based on refrigerant properties, theoretical cycle analysis was done to reveal the impact of evaporator, gas cooler and compressor, on the system cooling performance. It is concluded that the increase of overall compressor efficiency or the decrease of gas cooler approaching temperature could greatly improve the COP of this CO2 MAC system.

An Infrared Thermography Based Method for Quantification of Liquid Refrigerant Distribution in Parallel Flow Microchannel Heat Exchanger

An Infrared Thermography Based Method for Quantification of Liquid Refrigerant Distribution in Parallel Flow Microchannel Heat Exchanger

Quantification of liquid refrigerant distribution in parallel flow microchannel heat exchanger using infrared thermography

This paper presents a method to quantify the distribution of liquid refrigerant mass flow rate in a parallel flow microchannel heat exchanger from the infrared images. Quantification is achieved by building the relationship between the liquid mass flow rate through each microchannel tube and the air-side capacity calculated from the infrared measurement of the wall temperature. After being implemented in a heat exchanger model, the quantification method is validated against experimental data. This method can be used for several types of heat exchangers: evaporators, condensers, gas-coolers, etc., also it can be applied to various heat exchanger designs: different inlet/outlet locations, different flow configurations, etc.

A REVIEW OF REFRIGERANT MALDISTRIBUTION

In recent years, conservation of energy has become a challenging issue in air-conditioning applications. In order to overcome this issue, many researchers have recommended the use of a Parallel Flow Condenser and a low Global Warming Potential and Ozone Depletion Potential refrigerant, such as R32, in air conditioning systems. However, PFC faces the critical challenge of flow maldistribution in the tubes. This literature review mainly examines the refrigerant maldistribution problem which has been investigated by previous researchers. It was found that many of the researchers did not properly analyse the influence of flow maldistribution profiles on the performance degradation of heat exchangers. In order to have a comprehensive analysis of tube-side maldistribution in Parallel Flow microchannel heat exchangers, it is recommended that the influence of the higher statistical moments of the probability density function of the flow maldistribution profiles on performance degradation be quantified. Additionally, R32 maldistribution should be analysed and compared with R410A, which is the current commonly used refrigerant in air conditioning units. Moreover, in order to have a realistic simulation of the effect of refrigerant flow maldistribution profiles on performance degradation of heat exchangers, the effect of superheat and sub-cooling must be analysed.

Numerical Investigation of Heat Transfer in Parallel Rectangular Microchannel Heat Exchanger

The axial heat conduction in parallel flow microchannel heat exchanger with rectangular ducts was numerically investigated, for laminar, 3-D, incompressible, and steady state flow of water. The governing equations, continuity, Navier-Stokes equations (momentum equations), and the energy equations for the hot and cold fluids were solved by using SIMPLE algorithm with finite volume method and FORTRAN code to obtain the temperature distribution for the two fluids and the separating wall between them. The results play an important role of the axial heat conduction on the effectiveness in parallel flow microchannel heat exchanger and the factors affecting the axial heat conduction are; Reynolds number Re, thermal conductivity ratio, aspect ratio ? and channel volume. Increasing of Re, , ? and channel volume each separately leads to increase the axial heat conduction and vice versa.

Experimental and Numerical Study on Dieless Bending of 2-Piece Parallel Flow Microchannel Heat Exchanger

Parallel flow microchannel heat exchangers have been increasingly applied in HVAC (Heating, Ventilation and Air Conditioning) field due to their characteristics, such as high efficient heat transfer performance, compact form and low cost. Production rationalization of the heat exchanger is required for realization of a competitive, light-weight and easy-to-assemble product. The heat exchanger is usually bent to L-shape so as to decrease the volume of air conditioner. To correspond with design changes for multi-generation use and to achieve good forming quality, a dieless forming technique is developed to offer flexible manufacturing. Using this method, heat exchangers can be formed with variable bending radius conveniently, which can reduce the mold cost and obtain high productivity. Meanwhile, product defects such as fin damage can also be reduced because there is no physical contact against tools in the bending portion. In the present study, the forming principle of dieless bending process is first stated by theoretical analysis. Then to verify the proposed forming mechanism, a NC dieless bending machine is manufactured to implement the bending experiment. Finally, based on the verified FE model, the bending process for the 2-piece heat exchanger has been improved. As a result, the dieless bending process has been successfully developed and can be applied to produce square-shape heat exchanger with variable bending radius.

Study of the axial heat conduction in parallel flow microchannel heat exchanger

In this paper the axial heat conduction in an isosceles right triangular microchannel heat exchanger is numerically investigated, for laminar, 3D, incompressible, single-phase, steady state flow. The behaviour of axial heat conduction in the separating wall under different conditions is studied. The solution was obtained by solving the continuity and Navier–Stokes equations for the hot and cold fluids by using the pressure-correction method to obtain the velocity distribution, and then the energy equations were solved for the two fluids and the separating wall simultaneously to obtain the temperature distribution. The governing equations are discretized using finite-volume and the hybrid differencing scheme with FORTRAN code was used. Various parameters that can have effect on the axial heat conduction were investigated.The results showed that, the axial heat conduction plays an important role in a parallel flow microchannel heat exchanger and the factors affecting the local and average axial heat conduction are; Reynolds number (Re), thermal conductivity ratio (Kr), hydraulic diameter (Dh), thickness of separating wall (ts) and channel volume. Increasing of Re, Kr and ts leads to an increase in the axial heat conduction while increasing of Dh and channel volume leads to a decrease in the axial heat conduction.

Experimental investigation of thermal model of parallel flow microchannel heat exchangers subjected to external heat flux

This communication documents the experimental investigation of the theoretical model for predicting the thermal performance of parallel flow microchannel heat exchangers subjected to external heat flux. The thermal model investigated in this communication is that previously developed by the authors of this communication; Mathew and Hegab [B. Mathew, H. Hegab, Application of effectiveness-NTU relationship to parallel flowmicrochannel heat exchangers subjected to external heat transfer, International Journal of Thermal Sciences 31 (2010) 76–85]. The validity of the theoretical model with respect to microchannel profile, hydraulic diameter, heat capacity ratio and degree of external heat transfer is checked. The microchannel profiles investigated are trapezoidal and triangular with hydraulic diameter of 278.5 and 279.5μm, respectively. The influence of hydraulic diameter is analyzed using trapezoidal microchannels with hydraulic diameters of 231 and 278.5μm. Experiments are conducted for heat capacity ratios of unity and 0.5 using the heat exchanger employing the trapezoidal microchannel with hydraulic diameter of 278.5μm for purposes of validating the model. Experiments are done for all heat exchangers for two different levels of external heat transfer; 15% and 30% of the maximum possible heat transfer. Irrespective of the parameter that is investigated the experimental data are found to perfectly match with the theoretical predictions thereby validating the thermal model investigated in this communication.

Heat transfer characteristics of parallel and counter flow micro-channel heat exchangers with varying wall resistance

Thermal behaviours of parallel and counter flow micro-heat exchanger are investigated numerically, with emphasis on the role of wall resistance and thermal entry effects. The influence of different parameters, such as, thermal resistance (R), Knudsen number (Kn), effectiveness (e) and Number of Transfer Units (NTU) are investigated. It is found that the temperature jump at the walls increases with increasing R and Kn. On the other hand, the NTU increases with decreasing R and Kn. Furthermore, for counter flow, thermally fully developed flow may not exist over a large section of the heat exchanger due to the heat conduction in the separating wall and the entrance region.

Modeling non-adiabatic parallel flow microchannel heat exchangers

This article analyzes parallel flow microchannel heat exchangers with non-adiabatic thermal condition imposed on the wall separating the fluids and the ambient. For purposes of analysis, a thermal model comprising of two governing equations, one for each of the fluids, is developed and analytically solved to obtain equations for calculating the axial temperature and thereby the effectiveness of both fluids. Provision is provided in these equations for the temperature of the ambient interacting with the hot fluid to be different from that associated with the cold fluid. Analytical equations for determining the heat transfer between the individual fluids and the respective ambient as well as that between the fluids are also developed in this paper. The temperatures/effectiveness of the fluids depend on parameters such as NTU, fluid heat capacities, thermal resistance between the individual fluid and the respective ambient and ambient temperatures. In addition, under certain operating conditions the phenomenon of temperature-cross is observed. Depending on the ambient temperatures, increase in NTU will continuously improve the effectiveness of one the fluids; the effectiveness of the other fluid over the same NTU range increases before peaking and subsequent decrease. Equations for determining the NTU at which this peak value in effectiveness occurs are also provided. An unbalanced flow microchannel heat exchanger with the hot fluid having the lowest heat capacity has better effectiveness than when the cold fluid has the lowest heat capacity.

Experimental Investigation of Heat Transfer Characteristics on a Gas-to-Gas Parallel Flow Microchannel Heat Exchanger~!2009-07-27~!2009-10-13~!2010-04-02~!

Heat transfer characteristics of a gas-to-gas parallel flow microchannel heat exchanger have been experimen- tally investigated. Temperatures and pressures at inlets and outlets of the heat exchanger are measured to obtain heat transfer rates and pressure drops. The heat transfer and pressure drop characteristics are discussed. The results show that experimental pressure drop is approximately ten times as large as theoretically estimated pressure drop. Geometric con- figuration of the heat exchanger dominates pressure drop characteristics. The conventional log-mean temperature differ- ence method and the constant wall temperature model proposed in our earlier work are applied to predict heat transfer rate of the parallel flow microchannel heat exchanger. Prediction accuracy of the log-mean temperature difference method is superior to that of the constant wall temperature model. Applicability of the log-mean temperature difference method de- pends on direction of fluid flow.

Application of effectiveness-NTU relationship to parallel flow microchannel heat exchangers subjected to external heat transfer

In this paper the thermal performance of parallel flow microchannel heat exchangers subjected to constant external heat transfer has been theoretically analyzed. Equations for predicting the axial temperatures as well as the effectiveness of the fluids of a microchannel heat exchanger operating under laminar flow conditions have been developed. In addition, an equation for determining the heat transfer between the fluids has also been formulated. Irrespective of the heat capacity ratio, for a specific NTU, external heating always decreases and increases the effectiveness of the hot and cold fluid, respectively. The opposite trend in the effectiveness of the fluids is observed when they are subjected to external cooling. Moreover, under unbalanced flow conditions (heat capacities of two fluids are not equal) the effectiveness of the fluids depended on the fluid with the lowest heat capacity. Among the two unbalanced flow conditions (heat capacities of the two fluids are not equal) the effectiveness of the fluids is greatest when the hot fluid has the lowest heat capacity. At a given NTU, reduction in heat capacity ratio improved the effectiveness of the fluids. Under certain operating conditions temperature cross over was observed in the heat exchanger.

Fully developed thermal behaviors for parallel flow microchannel heat exchanger

The hydrodynamics and thermal behaviors of the flow in micro-heat exchanger parallel-plate are investigated numerically. The model that combines both the continuum approach and the possibility of slip at the boundary is adopted in the study. Effects of different parameters, such as, Knudsen number ( Kn), heat capacity ratio (Cr), the effectiveness ( e) and number of transfer units (NTU) are investigated. It is found that both the velocity slip and the temperature jump at the walls increase with increasing Kn. On the other hand, at low values of Kn, the NTU increases with increasing Cr, but at high values of Kn the NTU decreases with increasing Cr. With regard to the effect of Cr on e, it is found that increasing Cr leads to a reduction in e and for all Kn.

Developing Adiabatic Two Phase Flow in Headers—Distribution Issue in Parallel Flow Microchannel Heat Exchangers

Heat exchangers with microchannels (hydraulic diameter less than 1 mm) are attracting significant attention lately. Performance of these heat exchangers mostly evaporators is greatly affected by imperfect distribution of a two-phase fluid to each channel. This paper presents an overview of the issue, showing basic options to evenly distribute single and two-phase fluid and the methods to determine maldistribution. Special attention is given to describing two-phase regimes in an adiabatic horizontal header with R134a as a fluid, describing a two-phase map with area of good distribution.