T.N. Wong

Closed-loop pulsating heat pipe

Flow visualization was conducted for the closed-loop pulsating heat pipe (PHP) using a charge coupled device (CCD). It was observed that during the start-up period, the working fluid oscillates with large amplitude, however, at steady operating state, the working fluid circulates. The direction of circulation for the working fluid is consistent once circulation is attained, but the direction of circulation can be different for the same experimental run. Phenomena such as nucleation boiling, coalescence of bubbles, formation of slug and propagation of inertia wave were observed in the closed-loop PHP. The findings showed that the meandering bends, uneven slug and plug distribution and non-concurrent boiling at the evaporator contributed to the driving and restoring forces for fluid circulation and oscillations.

Numerical and experimental studies of refrigerant circuitry of evaporator coils

Abstract This study attempts to analyze the performances of evaporator coils with complex refrigerant circuitry using a distributed simulation model, which has three elements: branch, tube and control volume. The governing equations for a control volume are presented in the paper together with the computer simulation procedure for branches, tubes and control volumes of a coil. The model predictions on four test coils are validated with experimental data collected under different airflow conditions using R134a as a refrigerant. Using this model, the heat transfer and fluid flow characteristics of the coils are studied. The study shows that while the thermal resistance of refrigerant side is comparable to that of airside, the coil comprehensive performance can be improved by changing the refrigerant mass velocity along the flow path. Compared with a common coil, using a complex refrigerant circuitry arrangement where the refrigerant circuits are properly branched or joined may reduce the heat transfer area by around 5% in coil design. A guideline is proposed for the refrigerant circuitry arrangements to improve the coil performance.

Optimisation of single and double layer counter flow microchannel heat sinks

In this paper, a single layer counter flow (SLCF) and a double layer counter flow (DLCF) microchannel heat sink with rectangular channels have been modelled by employing the thermal resistance network with the correlations available in the open literature to evaluate the performance of the heat sinks. The accuracy of the prediction has been verified by comparing the results obtained with those from the more comprehensive three dimensional computational fluid dynamics (CFD) conjugate heat transfer model. Good agreements were obtained. The thermal resistance models has been linked with a multivariable constrained direct search optimisation algorithm to optimise the performance of the SLCF and DLCF microchannel heat sink under a set of design constraints. Optimisation results show that both the SLCF and DLCF heat sinks operating in the laminar out performed the heat sinks under turbulent flow conditions, both in heat transfer and hydrodynamic considerations.

Comparison of one-dimensional and two-dimensional models for wet-surface fin efficiency of a plate-fin-tube heat exchanger

Abstract This paper investigates the wet-surface fin efficiency of a plate-fin-tube heat exchanger. Depending on the compromise made between accuracy and complexity, a one-dimensional analytical model, a one-dimensional numerical model and a two-dimensional numerical model are proposed respectively for fin efficiency together with the corresponding computation algorithms. The one-dimensional numerical model considers the local mass transfer effect. The two-dimensional model takes into account of the complex fin geometry and the variation of moist air properties over the fin. In the analytical model, two parameters M∗ and T ∗ a are employed in the solution in place of the C parameter that is commonly used in the literature. A quantitative comparison of these models together with a widely used method by McQuiston is presented. Meanwhile, the fin performance of a plate-fin-tube heat exchanger is investigated under a variety of fin geometric parameters and airflow conditions, especially for a wide range of air relative humidity (from 20 to 95%). Due to the different simplifications made in each model, the differences among the calculation results are found to be significant. Finally, the applications and limitations of these models are discussed along with the error analysis.

A computer simulation of a rotary compressor for household refrigerators

Abstract This paper presents analytical studies of a fractional horse-power rotary refrigeration compressor and its performance comparison with measured results. The study employed a general-purpose performance model by considering the thermodynamic cycle of the compressor and its mechanical losses. The real gas equation of state is used to describe the changes of state of the refrigerant. The study reveals that for this quarter horse-power compressor unit the energy consumption in performing the compression cycle was 80–90% of the compressor shaft energy input, and the other 10–15% of the energy was dissipated as mechanical friction. The results also show that the performance predictions are satisfactory when compared with measured results. The model has been used in assisting the design of new compressors for use with new environmentally friendly refrigerants. This study also paves the way for more comprehensive simulation studies and for possible overall computerised optimisation design study in the future.

Numerical modeling of two-phase refrigerant flow through adiabatic capillary tubes

Abstract Literature shows that the homogeneous flow assumption has been commonly used in most of the adiabatic capillary tube modeling studies due to its simplicity. The slip effect between the two phases was often not considered in this small diameter capillary tube. This paper attempts to exploit the possibility of applying the equilibrium two-phase drift flux model to simulate the flow of refrigerant in the capillary tube expansion devices. Attempts have been made to compare predictions with experimental results. The details flow characteristics of R134a in a capillary tube, such as distribution of pressure, void fraction, dryness fraction, phase’s velocities and their drift velocity relative to the center of the mass of the mixture are presented.

Adiabatic capillary tube expansion devices : A comparison of the homogeneous flow and the separated flow models

Abstract Literature shows that the homogeneous flow assumption has been commonly used in most of the adiabatic capillary tube modelling studies. The slip effect between the two phases was often not considered in this small diameter (about 1 mm) capillary tube. Due to the lack of experimental information of the slip ratio in the flow within the capillary tube, the more comprehensive separated two-phase flow model was not frequently used in theoretical studies. This paper attempts to exploit the possibility of the separated flow model. Comparisons of the predicted results between the homogeneous and the separated flow models are presented, together with experimental results from previous workers. The results show that the separated flow model using the Miropolskiys slip ratio combined with Lins frictional pressure-gradient correlations gives a better prediction compared to the homogeneous flow model.

Refrigerant flow in capillary tube: An assessment of the two-phase viscosity correlations on model prediction

Abstract The homogeneous flow model has been widely used to analyse the two-phase flow of refrigerant in a capillary tube of a vapour compression refrigeration system. However, to effectively apply the model, it is necessary to use an appropriate two-phase friction factor with a suitable two-phase viscosity correlation. In this paper, the effects of the various two-phase viscosity correlations on the homogeneous flow model prediction are assessed by comparing with the predicted pressure drops along the capillary tube with measured data.

Analytical study of evaporator coil in humid environment

Abstract In this paper, a distributed simulation model for predicting steady-state performance of a direct-expansion air-cooling coil is developed. This model uses a numerical method to calculate the partially wet and totally wet fin efficiency and takes into account the refrigerant pressure drop along the coil. The model simulation of a test coil is validated with experimental data collected under different air conditions using R134a as a refrigerant. On the basis of this model, a number of parameters which reflect the characteristics of evaporator coils are analyzed in diverse humid environments and it is found that the performance of the coil is significantly affected by air relative humidity. Comparison of coil performance with R134a and R12 as refrigerants, whose difference in properties is small, shows that the differences in coil design for a specified cooling load and in the heat exchange characteristics for a given coil cannot be ignored.

A study of multiple heat sources on a flat plate heat pipe using a point source approach

Abstract An analytical approach has been employed to study liquid flows in an isotropic wick structure of a flat plate heat pipe with multiple heat sources. In this study, the heat sources have been modeled as point sources using the Dirac Delta function to describe the heat distribution. The two-dimensional pressure and velocity distributions are shown and discussed. The study has been extended to locate the positions of the multiple heat sources for optimum heat pipe performance. The optimum performance of the heat pipe is accomplished when the minimum pressure drop is attained across the wick structure.