Mass Measurement, Visualization and Signal Pattern Based Calibration of Capacitive Sensors for Measuring Void Fraction in Headers

This paper presents the calibration procedures of the capacitive sensors for measuring void fraction in headers with R134a. The sensors utilize the difference of dielectric constants between the liquid and vapor phases of the two-phase mixture. The 3D printed header has eleven microchannel-tube protrusions. Eleven capacitive sensors are located between protrusions. All sensors are made to be identical as much as possible in the manual fabrication procedures. Because the electric field within each sensor is not perfectly uniform, sensors need to be calibrated before measuring void fraction. Another transparent 3D printed header with the same inner geometry is installed at the same location of the facility for visualization and pressure drop measurement purposes. By controlling valves, the flow is directed to either header. The calibration is based on three methods: mass measurement (quick-closing valves, QCV), visualization (with a high-speed camera), and capacitive signals obtained from eleven individual sensors. After the calibration procedure, all sensors are ready to measure the void fraction in vertical headers

Experimental Study on Microchannel and Round Tube Plate Fin Evaporators in a Residential Air Conditioning System

The performance of two evaporators (round tube plate fin and microchannel) and their effect on system performances were experimentally investigated in R410a residential air conditioning systems. Both systems share identical components except evaporators and system performance were measured under AHRI A, B and C conditions. Under all test conditions, the system with the microchannel evaporator outperformed the baseline system with the round tube plate fin evaporator. Up to 10.6% increase of capacity and 13.1% increase of COP were achieved if both systems were operated under the same compressor speed. Up to 23.8% improvement of COP was obtained if the cooling capacities of both systems were kept the same. The refrigerant charge was reduced by 13.4% compared with the baseline system, due to the smaller internal volume of the microchannel evaporator. The air side heat transfer area of microchannel evaporator was just ½ of the baseline round tube plate fin evaporator, and so was the face area

Visualization and Simulation of Oil Flow in a Scroll Compressor Plenum

Discharge valves located in the compressor plenum are the gateway for the lubrication oil to leave the compressor to the system. The space of compressor plenum has the potential to separate oil droplets which can decrease the oil circulation ratio. In this paper, video processing method is used to capture the oil droplets distribution near the discharge valve and in the discharge tube in a running system. Discrete phase model in computational fluid dynamics is used as the numerical approach to study the oil flow inside compressor plenum. To validate the CFD simulation, size distribution from video processing at the valve is used as the input of the calculation and the calculation results are compared to size distribution in the discharge tube. Calculation shows how oil droplets are size distribution affect the oil flow in the plenum.

The Performance of an Automotive Carbon Dioxide Heat Pump System in Frosting and Defrosting

This paper presents the performance in frosting and defrosting of an automotive carbon dioxide (CO2) heat pump system, which is composed of a microchannel indoor and outdoor coil, a reciprocating compressor, an electronic expansion valve, and an integrated accumulator and internal heat exchanger. The effects of the criterion for the start of defrosting on the heat pump system performance were investigated experimentally. The defrost-start criterion was determined based on the reduction in the performance of the heat pump system instead of only considering the performance of the outdoor coil. Results show the heat pump system can operate for 64 minutes at 0℃ and 90% RH with a defrost-start criterion of 10 times the initial air-side pressure drop. The accumulator and internal heat exchanger improve system efficiency during frosting conditions. The mass of frost accumulation increases faster in the first 40 minutes and slower in the next 24 minutes for 0℃ and 90% RH. The retained water on the outdoor coil gets saturated after three short frosting/ defrosting cycles or a 64-minute long run. Also, the frosting and defrosting processes are monitored by a web camera to investigate the frost distribution on the two-pass outdoor microchannel heat exchanger. The results are presented for evaluating the frosting distribution and the effects on the overall performance of the heat pump system

Heat Transfer of Condensation in Smooth Round Tube from Superheated Vapor

Heat transfer of R134a condensing in a horizontal smooth round tube with 6.1 mm inner diameter is invested in this study. Experimental results on the heat transfer coefficient (HTC) with mass flux from 50 to 200 kg m-2 s-1 and heat flux from 5 to 15 kW m-2 are provided. Heat transfer behavior is compared between condensing superheated (CSH) region and two-phase (TP) region. Experimental result shows that in TP region, higher mass flux results in higher HTC while heat flux does not affect HTC. In CSH region, however, mass flux does not affect HTC while HTC increases when heat flux increases. Based on the flow characterization, the heat transfer behavior in CSH region is contrary to that suggested by liquid film geometry, where higher mass flux means thinner film whose HTC is higher and heat flux does not alter HTC a lot. In addition, the consistent peak of HTC at quality one cannot be physically explained while the reason for the counterintuitive heat transfer behavior actually lies in the conventional definition of HTC. Therefore, a new definition of heat transfer coefficient, described as “film heat transfer coefficient†is proposed to better represent the physics and serve as a preparation for the new heat transfer model

Numerical Investigation of Carbon Dioxide and POE Oil Mixture Behavior in an Accumulator in Trans-critical Heat Pump Mode

Carbon dioxide is one of the most promising next-generation refrigerants for air-conditioning applications in electric vehicles because it has the advantages of good heating performance in cold climates and environmental-friendly properties. Nonetheless, there are limited data about reversible CO2 heat pump systems for component-level performance and design. This paper focuses on the impact of different sizes of oil bleed hole on the liquid level and local pressure drop in the accumulator through CFD simulation. Also, we applied the experimental data obtained with a full CO2 HP system for the boundary conditions and the measured properties of liquid POE-CO2 for the materials in the simulation. The cases with different sizes of the oil bleed hole are investigated using the commercial computational fluid dynamics software Fluent. The results of this paper can be used to design the accumulator and improve 1-D models for reversible systems so that it can operate well in both air-conditioning and heat pump modes

Separation of Liquid and Vapor in Header of MCHE

This paper presents the experimental study of separation of two-phase flow in a vertical header of microchannel heat exchanger (MCHE) based on quantified visualization using fast camera, modelling analysis and experimental evaluation. A condenser model is developed to explore separation effects on heat exchanger. The modeling results show the benefits that a separation condenser has over a conventional condenser is affected by the separation results in the header. A header prototype is made that has an inlet in the longitudinal center part. Two sub-passes downstream are incorporated, lower for liquid and upper vapor flow. The header for experiment is clear to provide visual access. R-134a is used as the fluid of interest and mass flux through the inlet microchannels is controlled between 55 kg/(m2s)-195 kg/(m2s). The experiment results indicate that ideal separation in that header can happen at low mass flux up to 70 kg/(m2s). Results are presented in function of liquid and vapor separation efficiencies (ηl, ηv). Two phase flow inside the header is analyzed to study the mechanisms for liquid-vapor separation.

CFD Simulation of R134a and R410A Two-Phase Flow in the Vertical Header of Microchannel Heat Exchanger

Refrigerant maldistribution in the header of microchannel heat exchanger (MCHX) may reduce MCHX capacity by up to 30% because it creates the unwanted superheated region, where the heat transfer is much lower than the two-phase region due to the lower heat transfer coeeficient of superheated vapor and less temperature difference between refrigerant and air. The objective of this study is to gain more insights on the two-phase flow in the header through CFD simulation based on experimental results. In the experiment, the two-phase R134a or R410A is circulated into the transparent vertical header through multi-parallel microchannel tubes in the bottom pass and exits through multi-parallel microchannel tubes in the top pass representing the flow in the heat pump mode of a reversible system. Visualization reveals that R410A distribution is related to the size of churn flow region in the header. The best distribution is at high mass flux and low quality. When mass flux is lower, the churn flow region is smaller and the distribution is worse due to lack of sufficient momentum to supply liquid to the top tubes. At higher quality (e.g. xin = 0.6 and 0.8), besides lacking sufficient momentum to lift liquid, the churn flow region is much smaller because the semi-annular flow appears at the bottom parts of the header. The liquid film in semi-annular flow bypasses the bottom exit tubes and makes these tubes get less liquid than other tubes in churn flow region. The experimental results are compared with CFD simulation for better understanding. The Eulerian-Eulerian model in the commercial software Fluent is used to conduct simulation. Good accuracy of CFD simulation is achieved by comparing with the experimental results. The CFD simulation shows more details of two-phase flow pattern in the vertical header. The pressure contour shows the pressure increases along the upward flow. It is due to the fluid is exiting and the flow is decelerating. The velocity vectors show that there is a swirling between each two tubes in the header, except for the last two tubes. The swirling creates non-uniform pressure distribution in the same cross-section plane. Therefore, the location where to measure the pressure in the header affecting the understanding of two-phase flow in the header significantly