Predrag S. Hrnjak

Single-Phase Pressure Drop Measurements in a Microchannel Heat Exchanger

Single-phase pressure drops inside the complex headers and parallel circuits of a microchannel heat exchanger were measured systematically, for the purpose of identifying and quantifying internal manufacturing defects. Results confirmed the Moody chart to be applicable for the submillimeter channel flows. Based on nonintrusive testing followed by destructive evaluation, two types of manufacturing defects were quantified: variation of microchannel port diameters and port blockage by brazing flux. A pressure drop model for the whole heat exchanger was developed, and predicted the pressure and mass flow rate distribution inside the heat exchanger, providing valuable insights for selecting microchannel tube dimensions and heat exchanger circuiting. Most important, simple nitrogen flow tests can now be used to help control the quality of the brazing process, by detecting the presence of blocked or deformed ports in the finished product.

Two-phase flow visualization of R134A in a multiport microchannel tube

Results are presented from a flow visualization investigation of a multiport microchannel tube using R134a, a medium pressure refrigerant. The study covers mass fluxes from 50-300 kg/s.m 2 and qualities ranging from 10-90%, with a 6-port microchannel tube with a hydraulic diameter of 1.5 mm under adiabatic conditions. The results from the flow visualization studies indicate that several flow configurations may exist in multiport microchannel tubes at the same time while constant mass flux and quality flow conditions are maintained. Based on these observations, development of a typical flow regime map does not appear to be an appropriate manner for describing the flow field if the flow conditions for each port are not known. Flow mapping of the fluid regimes in this multiport microchannel is accomplished by developing functions that describe the fraction of time or the probability that the fluid exists in an observed flow configuration. An application of the alternative method of flow description for pressure drop predictions in multiport channels is included.

Flow-Regime-Based Model for Pressure Drop Predictions in Microchannels

A flow-regime-based pressure drop model is developed to correlate experimental data in the intermittent and annular flow regime. For the intermittent region, an alternative method for microchannel pressure drop predictions based on the average kinetic energy of the mixture is developed. A parameter based in the Weber number, the Lockhart-Martinelli parameter, and the liquid-to-vapor density ratio is proposed for pressure drop predictions in the annular flow region. The two correlations were built on experimental data of R-410A, R-134a, and air-water mixtures with mass fluxes ranging from 50 to 300 kg/(s·m2), with quality ranging from 0 to 1. Six-port and fourteen-port aluminum microchannel tubes with hydraulic diameters of 1.54 mm and 1.02 mm, respectively, were used for the two-phase pressure drop experiments. Entrance/exit pressure drops were directly measured with an additional experiment. The losses due to the entrance/exit zones have been found to exhibit homogeneous flow characteristics, and were correlated to a homogeneous flow parameter based on the kinetic energy of the flow field.

Two-Phase Refrigerant Maldistribution in the Vertical Header and its Effect on the Heat Exchanger Performance as Evaporator in Heat Pump Mode

The vertical header is a usual feature of outdoor heat exchanger of the residential air-conditioning system, typically a multipass microchannel heat exchanger. When operating in heat pump mode, it functions as an evaporator. In such a system, refrigerant maldistribution in the header can deteriorate the performance of the heat exchanger. The objective of study in this paper is the adiabatic upward flow of the refrigerant in the second pass 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. Three circular headers were explored, each with the microchannel tubes inserted to half depth. The best distribution is found at high flow rate and low quality. The distribution is affected by the flow patterns in the header as well as axial momentum. A distribution correlation, obtained based on the flow rate measurement in each tube, was then incorporated with a microchannel heat exchanger model. The simulation results quantified the capacity reduction of the 2-pass microchannel condenser due to the refrigerant maldistribution.Copyright

A Sensor for Estimating the Liquid Mass Fraction of the Refrigerant Exiting an Evaporator

A traditional method of controlling evaporator superheat in a vapor compression air conditioning system is the thermostatic expansion valve (TXV). Such systems are often used in automotive applications. The TXV depends on superheat to adjust the valve opening. Unfortunately, any amount of superheat causes that evaporator to operate at reduced capacity due to dramatically lower heat transfer coefficients in the superheated region. In addition, oil circulation back to the compressor is impeded. The cold lubricant almost devoid of dissolved refrigerant is quite viscous and clings to the evaporator walls. A system that could control an air conditioner to operate with no superheat would either decrease the size of its existing evaporator while maintaining the same capacity, or potentially increase its capacity with its original evaporator. Also, oil circulation back to the compressor would be improved. To operate at this two-phase evaporator exit condition a feedback sensor would have to quantify the quality of liquid mass fraction (when the exit stream is a mixture of droplets and superheated vapor) of the refrigerant exiting the evaporator.

Oil flow at discharge valve in a scroll compressor

This work focuses on high-speed visualization of the three reed valves in a scroll compressor. The visualization was performed to obtain qualitative description of the oil flow in terms of morphology and breakup and to quantify the droplet size and velocity distribution functions both for startup and cyclic operation conditions. It was observed that at startup, part of the oil film between the valve and its seat is first pushed out before the valve can start moving, after that the valve starts opening and the film is stretched so that liquid columns in organized spacing appear and are blown out by the gas originating the droplets that form a mist. During steady operation of the compressor, no oil film breakup was clearly observed at the valve–seat interface, suggesting that the oil droplets might be generated from vapor shear through the orifice since there is a recess that can hold up oil inside the orifice.

Measurement and visualization of R134a 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.

Using change point analysis for image processing of developing adiabatic two-phase flow after expansion valve

This paper presents a method of analyzing images obtained from a high speed camera in a study of developing two-phase refrigerant flow after an expansion valve, using a technique known as Change Point Analysis. Specifically, a method for automating the determination of separation distance and the location of the liquid/vapor interface in stratified type flows is outlined. The experiments were conducted in transparent PVC tubes, with inner diameters of 8.7mm and 15.3mm. A working fluid of oil free R134a was used. Dynamic flow parameters of inlet quality and mass flux were varied from 5–35% and 15–40g/s, respectively. Flow visualization was achieved through the use of a high speed camera.© 2007 ASME

Online Measurement Techniques for Determining Oil Circulation Rate

Refrigeration and air-conditioning (AC) systems employ refrigerant as the working fluid; however, a portion of oil is discharged from the compressor as part of the compression process and also circulates through the system. This small amount of parasitic fluid causes heat transfer and pressure drop correlations that were developed for pure refrigerant flow to fail and needs to be determined for proper design of heat exchange equipment and connection piping. It is desired to be able to measure the small concentrations of oil circulating as a component of the working fluid online in real time. The oil in circulation as a fraction of the total fluid flow rate is termed the oil circulation rate or oil circulation ratio (OCR). The goal of this study was to determine which combination of fluid property measurements could be used to accurately and precisely quantify OCR. Oil, which is needed to lubricate the compressor, is carried with the refrigerant throughout the system. Oil affects fluid properties such as enthalpy, thermal conductivity, and viscosity and can impact the ability to accurately measure heat exchanger and system performance. Fluid property and flow maps have been developed for various refrigerant-oil mixtures; in combination with these maps the ability to accurately measure OCR online may prove to be a powerful tool in quickly measuring, analyzing, and improving system performance. Without this ability to accurately measure the oil circulation rate over the range of operating conditions, it is impossible to create accurate thermodynamic balances based entirely on the properties of the refrigerant portion of the working fluid. The refrigerant-lubricant mixture selected for this study is a commonly used mixture for automotive AC systems: R134a with ND-8 oil. In a typical air conditioner, utilizing R134a with ND-8, a single phase exists only in subcooled portions of the condenser and the liquid line. Therefore, the experiments were conducted at typical automotive AC conditions between 20 °C and 45 °C, pressures ranging from the saturation pressure up to 1900 kPa, and an OCR between 0% and 12%, and a fixed mass flux of nominally 300 kg/m2 s. For a single phase fluid comprised of two components, it is necessary to measure three independent fluid properties to completely describe its state. Since the temperature and pressure are easily obtainable, additional readily available properties to determine the liquid composition were selected: density, ultra-violet light absorptivity, and refractive index. The accuracy and precision of calculating the OCR with these measurements are compared analytically and experimentally. The experimental apparatus was located within an environmental chamber which was capable of controlling the temperature over the range of test conditions. The working fluid was circulated using an oil free gear pump and the pressure of the mixture was controlled via a hydraulic cylinder which was attached to a variable pressure source. Precise quantities of oil were incorporated into the working fluid with a high pressure liquid chromatography pump. A length of clear nylon tubing permitted flow visualization.Copyright

Thermodynamic Analysis of the Transition Between Slug and Annular Flow in Minichannels

Heat transfer mechanism during flow boiling of fluids in small channels differs significantly depending on whether two-phase flow is in slug or annular regime. Understanding of the transition conditions between homogeneous slug flow and annular two-phase flow is an important topic for mini- and microchannel heat exchangers performance optimization. The current study focuses on the analysis of thermodynamic equilibrium conditions of two neighboring two-phase flow regimes. In both flow patterns the total energy is equal at specific mass flux and vapor quality and those values can be used to mark the transition conditions.Copyright