Brandon S. Field

Adiabatic Two-Phase Pressure Drop of Refrigerants in Small Channels

The adiabatic pressure drop of two-phase refrigerant flow in small channels has been investigated. A rectangular channel with d h = 148.0 μm has been tested with four refrigerants: R134a, R410A, propane (R290), and ammonia (R717). These data have been combined with data taken from five different channels, with d h varying from 70 μm to 305 μm, of R134a. The measured pressure drops have been compared to many published separated-flow and homogeneous pressure drop models. A new correlation for C, the Chisholm parameter, has been developed based on the Reynolds number of the vapor phase (which contains the majority of the kinetic energy) and the dimensionless grouping ψ—a ratio of viscous to surface tension effects taken from the analysis of capillary flow performed by Sou and Griffith (1964). This allows the new correlation to account for the varying fluid properties (including surface tension) that are found in the different refrigerants. The new correlation takes flow regime into account by means of a Weber number based flow transition criteria, following the flow map of Akbar et al. (2003).

An Air Curtain Along a Wall With High Inlet Turbulence

A downward blowing isothermal wall jet at moderate Reynolds numbers (1,500 to 8,500) with significant inflow turbulence (ca. 6%) was investigated. The flow configuration is an idealization of the air curtains of refrigerated display cases. Flow visualization using particle seeding was employed to identify the flow field eddy dynamics. Particle Image Velocimetry was used to examine the velocity fields in terms of mean and fluctuating values

Visualization of two-phase refrigerant and refrigerant-oil flow in a microchannel

Visualizations of adiabatic two-phase refrigerant flow in a glass channel of diameter 0.5 mm have been made for three refrigerants: R134a, Propane (R290), and Ammonia (R717), representing a wide span of fluid properties, which covers most of the refrigerants commercially in use. In these visualizations four flow regimes were observed: bubble-slug, slug, slug-annular, and annular. These flow regimes were compared to various flow maps, including some developed for small channels. Flow visualizations were also made with mixtures of R134a and 68-weight POE oil at oil circulation rates of approximately 0.5, 1.5 and 3 percent. This is of interest when considering refrigeration systems, which have a small percentage of oil in circulation that travels through system and through the heat exchangers. When the refrigerant is in a liquid state, this presents little variation in fluid properties, because the concentration is so small. However, when the refrigerant is partly vapor, the oil concentration in the remaining liquid can have significant effect on the fluid properties. In addition, the saturation temperature and pressure of the oil-refrigerant mixture changes with concentration, where a single-phase vapor is never observed in flows of oil-refrigerant mixtures, even at temperatures exceeding the saturation temperature of the pure mixture. This effect is known as “apparent superheat”.Copyright

A mechanistic model of two-phase pressure drop in microchannels

A mechanistic model of two-phase pressure drop has been developed for microchannel flow. The primary flow regimes observed in microchannel two-phase flow regime maps were the inertial dominated regime (annular flow) and the surface tension dominated regime (slug or bubbly flow). Mechanistic models of pressure drop for each of these regimes are developed and compared to pressure drop in microchannel flows of four different refrigerants with widely varying fluid properties: R134, R410A, R290 (propane) and R717 (ammonia) of varying hydraulic diameters between 70 microns to 305 microns. The mechanistic model compares favourably to the experimental pressure drop data from microchannel flow measurements with an overall mean deviation of 18.1%. The model is also compared with refrigerant-oil flow of R134a and two different weights of POE oil, using fluid property correlations developed for those mixtures, with a mean deviation of 18.9%. While this accuracy is not stellar, the significance is that no empirical coefficients were needed to close the model.Copyright

Guided Inquiry Learning in a Thermodynamics Class

This paper describes guided inquiry investigations of thermodynamic properties and cycles that was used in a sophomore thermodynamics class. A partially-complete computer model of a Carnot cycle was provided to students that was written in Engineering Equation Solver (EES). The students were led through a investigation in pairs or groups using the model designed to familiarize the students with the software as well as the cycle. As part of the investigation, the students were required to modify the model to add features. Following the in-class exercise with the Carnot cycle, students were required to model a Stirling cycle and later a Brayton cycle using the same EES software for a take-home project. In previous classes, the same (and similar) take-home projects were assigned, but this was the first time that these computer projects were preceded by a guided inquiry investigation. The advantages of using such a guided inquiry investigation to introduce the software were primarily evident from the questions that were avoided on the take-home projects, but also evident from the student performance on the projects themselves. A discussion of the investigation is included, as well as critique of what will be changed for the next time the class is taught.Copyright

Visualization of Dynamic Contact Angle

Capillary rise of air-water-solid systems have been recorded with high-speed video. Glass and metal have been used as the solid phase, and the dynamic shape of the meniscus and contact angle have been characterized. The advancing and receding contact angle is of interest in computational simulations of boiling flow, and the present visualizations attempt to quantify the dynamic aspects of contact line motion. The centroid of the capillary meniscus has been tracked in order to determine the force at the contact line based on a force balance of the elevated fluid phase. The solid phase is raised and lowered in the fluid at different rates to observe advancing and receding contact lines.Copyright

Integrated Thermal Conduction and Hardenability Laboratory Activity

This paper describes an integrated laboratory project between separate heat transfer and machine design courses. The project was structured around a Jominy end quench hardenability test. Most of the students participating were simultaneously enrolled in both classes. In the heat transfer class, students were required to model one-dimensional, transient thermal conduction for an end quench geometry of 4140 steel. In machine design, students applied their theoretical temperature profiles to a continuous cooling transformation curve (CCT) of 4140 steel to predict microstructure and matched the theoretical cooling rates with hardenability curves from literature to predict hardness. In laboratory, students then performed an end quench test in accordance with ASTM A255 on four steel rods. By combining activities across the two courses, students developed an appreciation for the interconnectivity of material within the engineering curriculum, and learned that practical applications typically require they employ knowledge from a variety of sources.Copyright

Analysis of Secondary Features From Visualization of Two-Phase Refrigerant Flow

Flow visualizations of refrigerant flows were made in a 0.5 mm glass channel. High speed videos of two-phase R134a flows were used to examine the secondary features within the two-phase flow and categorize the flow regimes present. Essentially, in small diameter channels, there are only two major observed flow regimes: inertia dominated (annular flow) and surface tension dominated (intermittent flow: slug or bubble). Image analysis was used to measure the velocities of the slugs and bubbles, and measurements of secondary features of the flow, such as the velocity of the surface waves that arose in slug-annular flow and the smaller bubbles that could be seen in the wakes of larger bubbles.The secondary features of the two-phase flow are discussed with regard to their contribution to the flow regimes that are observed as the flow moves through the channel. Rings of liquid were observed to form into liquid slugs, dividing the longer bubbles into two. This is believed to be one of the transition mechanisms between the two flow regimes. Small droplets were also observed within the vapor core, which demonstrated velocity gradients within the vapor bubbles. Velocity measurements of the secondary features within the liquid films are used in the analysis of the dynamics within these flows that lead to pressure drop and flow regime transition.Copyright

Visualization of Two-Phase Refrigerant Flow Through an Ice-Maker Evaporator

A transparent evaporator plate has been affixed to a commercial ice-maker refrigeration system and high-speed visualization of the two-phase boiling of the R134a has been made as water is frozen on the top of the plate. The startup transient period of the freeze cycle, characterized by superheated vapor throughout the evaporator, can last up to 10 minutes as the water is pre-cooled, and the freeze cycle follows. The two-phase flow patterns have been observed as distance along the evaporator plate during the freeze cycle, and observations about the liquid phase distribution are made. The refrigerant path is a series of spiral turns, which sets up a flow pattern of the liquid phase gathering along the inner wall of the channel and being dragged by the high speed vapor. This is not an optimal flow pattern for heat transfer because the liquid is not distributed to the top of the channel.Copyright