An Experimentally Validated Model for Microchannel Heat Exchanger Incorporating Lubricant Effect

In the microchannel heat exchanger model that is developed in this study, the thermodynamic and transport properties of refrigerant-oil mixture are taken into account as well as their impact on boiling heat transfer and pressure drop. This model is validated against experimental results (R134a-PAG 46 oil) at various oil circulation rates (0.1%-8.3%). The agreement between measurement and prediction is ±4% for capacity, ±10% for pressure drop and ±3? for superheat at compressor inlet. Inclusion of lubricant in the new model has provided better prediction over models using pure refrigerant. Simulation results also indicate that lubricant addition improves refrigerant distribution, thus decreases the capacity degradation due to maldistribution

Void Fraction and Flow Regimes of R134a In Horizontal and Vertical Round Tubes in Developed Adiabatic Conditions

This paper presents flow regimes and void fraction in horizontal and vertical round tubes ID 7 mm with R134a in the adiabatic conditions and low mass flux (40-150 kg/m2s for horizontal tubes and 65-115 kg/m2s for vertical tubes) captured by a high-speed camera. Horizontal flow patterns are compared to Wojtan-Ursenbacher-Thome flow regime map and some modifications are proposed. Void fraction results for both horizontal and vertical tubes are compared to some widely used correlations. Influences of tube orientation and mass flux on void fraction are discussed. At the same vapor quality condition, void fraction of horizontal tubes is larger than that of vertical tubes. Higher mass flux also results in larger void fraction compared that of lower mass flux

Pressure Drop Of Condensation From Superheated Vapor Inside Horizontal Smooth Round Tubes

Pressure drop of R134a, R32 and R1233zd(E) is measured and reported in diabatic conditions during condensation from superheated vapor inside horizontal smooth round tubes. The test conditions include mass fluxes from 100 to 400 kg m-2 s-1, heat fluxes from 5 to 15 kW m-2 and tube diameters of 4 and 6 mm at saturation temperatures of 30 oC. Compared to a conventional pressure drop model constructed under the assumption of thermal equilibrium, the experimental data clearly shows that the onset and end of condensation, instead of being fixed at bulk quality 1 and 0, actually is changing according to the test conditions. This discrepancy between the theory and reality results in a deviation between the prediction and data, especially in the condensing superheated (CSH) region. The result shows an increase in pressure drop as mass flux increases. Tube size also affects the pressure drop in that smaller tube yields higher pressure drop. A further comparison between the different refrigerants conditions illustrates the effects of properties such as liquid-vapor density ratio, liquid viscosity, surface tension and latent heat. The results are analyzed along with the visualizations of the flow. While viscosity and velocity gradient determines the magnitude of pressure drop, the waves on the interface and the velocity of the bulk flow are identified as the two competing factors when condensation proceeds. The process as described in this paper provides an insight for a mechanistic model that traces the development of the flow to help resolve the issues in conventional pressure drop models

Pressure Drop Model For Condensation From Superheated Vapor

A new pressure drop model based on flow regime map is proposed for condensation inside horizontal smooth round tubes accounting for the non-equilibrium in a vapor compression system. Conventionally, a pressure drop model for two-phase flow only accounts for the prediction between bulk quality 1 and 0. The temperature gradient during condensation, however, creates the non-equilibrium that guarantees two-phase flow beyond bulk quality 1 and 0. The new model determines the onset and end of condensation by tracing the development of the liquid film when the superheated vapor is condensed on the tube wall. The flow regime map designed specifically for condensation from superheated vapor is used to predict the flow regime when the flow is two-phase. Two flow regime transitions are recognized. One is from annular flow to the stratified flow under low mass fluxes; the other is from annular flow to the intermittent flow under high mass fluxes. The annular flow is treated as a uniform annular ring; the stratified flow is treated as a combination of annular flow on the upper part of the tube and liquid pool at the bottom part of the tube; the intermittent flow is treated as a combination of annular flow and single-phase liquid flow that occur intermittently. The weights designated to each flow regime is calculated from the void fraction model used in the flow regime map. The construction of the new model is guided by the flow visualizations of several different refrigerants under various working conditions in tubes with diameters of 4 and 6 mm. The prediction of the new model is compared with experimental data of R32, R134a and R1233zd(E) mass fluxes from 100 to 400 kg m-2 s-1, heat fluxes from 5 to 15 kW m-2 and tube diameters of 4 and 6 mm at saturation temperatures of 30 oC. The comparison shows that the new model provides good agreements with experimental data. Additionally, by accounting for the non-equilibrium in the condensation process, the new model seamlessly connects the single-phase and two-phase regions with the corresponding mechanisms that occurs in a real vapor compression system

Linked Modelling of Heat Pump Water Heater Vapor Compression System and Water Tank

A computational fluid dynamics (CFD) model is developed in ANSYS Fluent for simulating the heat transfer and fluid dynamics in the water tank of a US-type residential heat pump water heating unit that utilizes a wrap-around coil condenser that transfers heat from refrigerant to water through the tank wall. The linked modelling process involves iteration between the CFD model of the water tank and a vapor compression system model (using EES) in describing quasi-steady warm up of a heat pump water heating system. The models are connected via temperatures and heat transfer at the tank wall. The models and the linking procedure are validated experimentally based upon a quasi-steady approach. A wholesome understanding of heat distribution and fluid dynamics in the water tank can enable design optimizations of coil geometry, placement (pitch), etc. Performance is influenced by profiles of water temperature in the tank and refrigerant in the condenser, assuming constant pressure drop, during various warm-up experiments

Intermediate Vapor Bypass: A Novel Design for Mobile Heat Pump at Low Ambient Temperature

With market share of electric vehicles continue to grow, there is an increasing demand of mobile heat pump for cabin climate control, as it has much higher energy efficiency than electric resistive heating and hence much less impact on electric drive range. However, current mobile heat pump systems using low pressure refrigerant like R134a and R1234yf suffer from significant heating capacity loss at low ambient temperature. As a result, a large electric heater needs to be installed to supplement the capacity shortage at low ambient temperature, and electric drive range can be greatly reduced due to large power consumption for cabin heating. In this paper, the drop of heating capacity at low ambient temperature was experimentally and numerically studied. Pressure drop and refrigerant maldistribution in the outdoor heat exchanger in HP mode were found to be the most important factors. A novel design of the outdoor heat exchanger using intermediate vapor bypass in HP mode and the corresponding system architecture were proposed. The proposed outdoor heat exchanger turns into a condenser with integrated receiver and subcooler in A/C mode. A proof-of-concept heat exchanger prototype was made by modifying the baseline heat exchanger and tested in the lab. The result has shown 35% improvement of heating capacity at -20 ̊C ambient condition. Optimization of the outdoor heat exchanger design was investigated with a system model

Measurement and Visualization of R410A Distribution in the Vertical Header of the Microchannel Heat Exchanger

This paper presents the refrigerant adiabatic upward flow in the vertical header of microchannel heat exchanger and its effect on distribution. R410A is circulated into the header through the microchannel tubes (5 or 10 tubes) in the bottom pass and exits through tubes (5 or 10 tubes) in the top pass representing flow in the heat pump mode of reversible systems. Three circular headers were explored, each with the microchannel tubes inserted to half depth. The quality was typically varied from 0.2 to 0.8. Mass flow rate was from 1.5 to 4.5 kg/h per microchannel. The best distribution is found at high flow rate and low quality. Distribution is improved by doubling the number of microchannel tubes although elongation of the header has negative effect. Visualization reveals the effects of flow patterns in terms of homogeneity and liquid momentum. Refrigerant in the churn flow has better distribution than in the separated flow since the two-phase mixture is more homogeneous. The distribution is better at high mass flux in the header because the higher momentum liquid can be supplied to the top exit tubes

Void Fraction and Flow Regimes Determined by Visualization, Mass Measurement and New Capacitance Sensor

This paper presents void fraction and flow regimes determined by three methods: visualization (high speed camera), mass measurement (quick-closing valves) and a newly developed capacitance sensor. In a way, this is a calibration process for a capacitance sensor. It is shown that new sensor can characterize flow patterns in low mass flux range and measure void fraction for horizontal and vertical tubes. A calibration procedure of void fraction measurement is based on a mass measurement (quick-closing valves). Two calibration curves for measuring void fraction in horizontal and vertical tubes are developed. With calibration curves, sensors with similar configurations can be directly utilized to measure void fraction in further studies

Mobile Heat Pump Exploration Using R445A and R744

The increased usage of hybrid and electric vehicles where waste heat availability is limited has spurred research and development of mobile heat pump systems. Many options exist for heat pump system architectures and refrigerants to be used. Currently R134a use is prevalent in vehicle air conditioning systems but offers poor heat pump performance at low ambient temperatures. Two refrigerants will be explored in this paper, R744 and R445A. Both of these refrigerants are getting attention in vehicle A/C systems because of their relatively low GWP but each offers benefits over R134a in heat pump systems as well. Both refrigerants operate at higher pressures which improves the performance at low ambient temperatures in part due to higher compressor inlet refrigerant densities. R134a (and R445A to a lesser extent) also suffer from the drawback of going into sub-atmospheric pressure operation at temperatures commonly seen in vehicles. Data will be shown for multiple system architectures comparing these refrigerants to R134a. Advantages and disadvantages of each refrigerant will be shown. System control and optimization is important to get the maximum performance from each refrigerant and system. Control exploration will be presented for each alternative refrigerant

Comparison and Generalization of R410A and R134a Distribution in the Microchannel Heat Exchanger with the Vertical Header

This paper explores the effects of fluid properties on two-phase flow in the vertical header and refrigerant distribution into horizontal branch tubes. R410A and R134a are used as the working fluid. Refrigerant enters into the header by five microchannel tubes in the bottom pass and exits through the five microchannel tubes in the top pass representing the flow in the outdoor microchannel heat exchanger of reversible systems under heat pump mode. The difference of fluid properties causes the flow pattern and refrigerant distribution results of R410A and R134a different between each other. Non-dimensional analysis shows that the inertia of R410A is higher than that of R134a. At low qualities, when the flow regime is churn, the higher inertia enables top tubes to receive more liquid so that the distribution of R410A is a little better than that of R134a. At high qualities, when it is likely semi-annular, the higher inertia causes more bottom tubes are bypassed by the liquid film of semi-annular flow. It results in worse R410A distribution than R134a, though more liquid reaches the top tubes for R410A. The coefficient of variation of refrigerant distribution is applied in this study to generalize the results of both R410A and R134a at various header geometries and inlet flow conditions. A distribution function is derived for predicting R410A and R134a distribution in future