Heat Transfer and Pressure Drop during Evaporation of R134a in Microchannel Tubes

The paper presents result for heat transfer and pressure drop in evaporation of R134a in microchannel tubes conducted in a facility with a 6 m long tube, modified to provide realistic situations for refrigerant blends with even the highest glide. The concept of the experimental facility is to measure heat transfer coefficient and pressure drop on the refrigerant side in condensation and evaporation with or without oil with heat exchanger in mind. The auto-controlled test line has 6 test sections for testing and 5 conditioning sections to preset the inlet quality of each test section. This facility provides data in the complete process of evaporation (quality from 0 to 1) or condensation (quality from 1 to 0) in a single pass. The secondary fluid in coolant loop for heating or cooling is water. By controlling the inlet water temperature of each test section, both constant wall temperature and constant heat flux conditions or anything in-between can be achieved. The tertiary loop is a chiller loop running with glycol/water mixtures to cool the water and refrigerant. First results with R134a in this facility show heat transfer coefficient and pressure drop changes with vapor quality and represent excellent starting point (baseline) for explorations of mixtures of low pressure and low GWP refrigerants that are replacements for R410A.

Heat transfer coefficient, pressure drop, and flow visualization of R1234ze(E), and R1234yf in microchannel tube

This paper presents heat transfer coefficient, pressure drop, and flow visualization of R1234ze(E) and R1234yf measured in the same facility as introduced in Li and Hrnjak (2016). Experiments are conducted on a 24-port microchannel tube with a hydraulic diameter of 0.643 mm. Mass flux varies from 100 to 200 kg m-2s-1. Heat flux varies from 0 to 6 kW m-2. A comprehensive presentation of measurement will be made in this paper. HTC, PD, and flow pattern will be reported in the same figure. The flow pattern results of R1234yf and R1234ze(E) are shown in the previous paper (Li and Hrnjak, 2018). With the capture of flow pattern, discussion of results are enhanced. The pressure drop of R1234ze(E) is higher than R1234yf. Heat transfer coefficient and flow pattern are similar of the two refrigerants. HTC increases as heat flux or mass flux increases. PD increase as mass flux increases or saturation temperature decreases

An experimental facility for microchannel research and evaporating R134a in microchannel tube

Microchannel research is important for heat exchanger design. Heat transfer coefficient and pressure drop are the two most important parameters to be measured in an experimental facility. A featured design of heat transfer behavior and pressure drop testing facility for both evaporating and condensing refrigerant in microchannel tubes are made and introduced in this thesis. Details of the installing of the test section, conditioning section, pressure sensors, and other important component of the facility are discussed in section 2. The calibration process of measurement instruments and results are reported in section 3. Heat loss to ambient, effect of thermal paste contact resistance, and temperature uniformity have been discussed in detail. The process of data reduction and uncertainty analysis are discussed. In section 4, testing results of the facility with evaporating R134a are given. Both heat transfer coefficient and pressure drop are measured and reported in this thesis. The conclusion gives a brief summary of this thesis. The appendix has included calibration data from section 2 and raw data from section 4

Flow Visualization of R134a, R1234ze(E), and R1234yf in microchannel tube

This paper presents visualization results of two-phase flow of R134a, R1234yf, and R1234ze(E) in microchannel tube with hydraulic diameter of 0.643 mm. Visualization section has been made to record the flow in the microchannel to study the flow characteristic. Flow pattern maps of R134a, R1234ze(E), R1234yf are reported in this paper. Flow pattern maps are presented in two ways: mass flux-quality as coordinates, and superficial velocities as coordinates. The quality range of experiment is from 0 to 1 thermally. Mass flux varies from 50 to 250 kgm-2s-1. Flow patterns are classified as: plug/slug flow, transitional flow, and annular flow. The quality of boundary between two flow patterns is lower when mass flux is higher. At low vapor superficial velocity, the flow is in plug/slug. The vapor fraction and the interface velocity in plug/slug flow agrees well to the homogeneous prediction. As the vapor velocity increases, the flow becomes transitional flow. At higher vapor velocity and high liquid-to-vapor velocity ratio, the flow becomes annular

Evaporation of pure fluids and mixtures in a microchannel tube

Two-phase flow in microchannel tube is taking huge interest by both the academic and engineering worlds. It is necessary to understand the flow when the diameter is at the scale of one millimeter. The decreasing in diameter enhances the heat transfer, but also increases pressure drop. The two-phase flow pattern in microchannel tube is simpler due to the importance of surface tension. However, existing flow pattern maps, heat transfer coefficient and pressure gradient correlations are not accurate enough in prediction. This study focuses on the measurements and modeling of two-phase flow in microchannel tube. Six pure fluids (R134a, R32, R1234ze(E), R1234yf, R1233zd(E), and R1336mzz(Z)) are tested, and they have significant difference in properties. The experiments are conducted in a 24-port microchannel tube with an averaged hydraulic diameter of 0.643 mm. The tested microchannel has hydraulic behavior as a round tube (fRe=64 in laminar flow). Heat transfer coefficient, pressure gradient, and flow patterns are measured or recorded simultaneously. Deep discussion is made for illustrating the two-phase flow in microchannel tube by connecting flow patterns to measurements. Measurements are done at varied mass fluxes, heat fluxes, saturation temperatures to understand the effect of the operating conditions. The results are compared with each other to discuss the effects of properties. Comparisons to existing predictive models are made and discussed. The flow patterns are also reported and compared to existing maps. Measurements in the video show the homogeneity in plug/slug flow when velocity is low. Novel video processing method is introduced for measuring vapor plug velocity in videos. A liquid droplet forming mechanism is reported. The liquid slug collides with liquid ring and breaks into several droplets in the vapor core. One commercial mixture (R448A) and three lab made mixtures (R32+R1234yf at 15/85, 50/50, and 85/15 mass fractions) experiments are also conducted on the same facility. Results are compared to the component fluids. The effect of the temperature glide on zeotropic mixtures is reported and discussed. A new flow pattern map based on force balance and kinetic energy analysis is introduced. The flow pattern map shows good agreement with the measurements. A mechanistic model based on liquid film thickness in annular flow is proposed. The model calculates void fraction, pressure gradient, and heat transfer coefficient at the same time and it has good agreement with the measurements

Evaporation of pure fluids and mixtures in a microchannel tube

Two-phase flow in microchannel tube is taking huge interest by both the academic and engineering worlds. It is necessary to understand the flow when the diameter is at the scale of one millimeter. The decreasing in diameter enhances the heat transfer, but also increases pressure drop. The two-phase flow pattern in microchannel tube is simpler due to the importance of surface tension. However, existing flow pattern maps, heat transfer coefficient and pressure gradient correlations are not accurate enough in prediction. This study focuses on the measurements and modeling of two-phase flow in microchannel tube. Six pure fluids (R134a, R32, R1234ze(E), R1234yf, R1233zd(E), and R1336mzz(Z)) are tested, and they have significant difference in properties. The experiments are conducted in a 24-port microchannel tube with an averaged hydraulic diameter of 0.643 mm. The tested microchannel has hydraulic behavior as a round tube (fRe=64 in laminar flow). Heat transfer coefficient, pressure gradient, and flow patterns are measured or recorded simultaneously. Deep discussion is made for illustrating the two-phase flow in microchannel tube by connecting flow patterns to measurements. Measurements are done at varied mass fluxes, heat fluxes, saturation temperatures to understand the effect of the operating conditions. The results are compared with each other to discuss the effects of properties. Comparisons to existing predictive models are made and discussed. The flow patterns are also reported and compared to existing maps. Measurements in the video show the homogeneity in plug/slug flow when velocity is low. Novel video processing method is introduced for measuring vapor plug velocity in videos. A liquid droplet forming mechanism is reported. The liquid slug collides with liquid ring and breaks into several droplets in the vapor core. One commercial mixture (R448A) and three lab made mixtures (R32+R1234yf at 15/85, 50/50, and 85/15 mass fractions) experiments are also conducted on the same facility. Results are compared to the component fluids. The effect of the temperature glide on zeotropic mixtures is reported and discussed. A new flow pattern map based on force balance and kinetic energy analysis is introduced. The flow pattern map shows good agreement with the measurements. A mechanistic model based on liquid film thickness in annular flow is proposed. The model calculates void fraction, pressure gradient, and heat transfer coefficient at the same time and it has good agreement with the measurements.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste