Kinga Strąk

Calculations of Flow Boiling Heat Transfer in a Minichannel Based on Liquid Crystal and Infrared Thermography Data

ABSTRACT This paper analyzes results concerning flow boiling heat transfer in two parallel, asymmetrically heated vertical minichannels. The heating element for FC-72 Fluorinert flowing in the minichannels was a thin foil with an enhanced surface on the side in contact with the fluid. In one minichannel, changes in the temperature on the smooth side of the foil were monitored using liquid crystal thermography. Changes in the temperature on the outer surface of the glass in one minichannel and on the foil in the other minichannel were observed using infrared thermography. The heat transfer coefficient at the foil–fluid interface was calculated on the basis of one- and two-dimensional heat transfer models. In the two-dimensional method, the distribution of temperature on the enhanced side of the foil was determined by solving the inverse heat conduction problem. The governing equations were solved using the finite-element method combined with the Trefftz functions used as shape functions. The temperature measurement points were located at the boundary nodes of elements. Local values of the heat transfer coefficient calculated with the one- and two-dimensional models were analyzed in the function of the distance from the minichannel inlet. The values obtained with the two models were similar.

A study of the flow boiling heat transfer in a minichannel for a heated wall with surface texture produced by vibration-assisted laser machining

The paper presents results concerning flow boiling heat transfer in a vertical minichannel with a depth of 1.7 mm and a width of 16 mm. The element responsible for heating FC-72, which flowed laminarly in the minichannel, was a plate with an enhanced surface. Two types of surface textures were considered. Both were produced by vibration-assisted laser machining. Infrared thermography was used to record changes in the temperature on the outer smooth side of the plate. Two-phase flow patterns were observed through a glass pane. The main aim of the study was to analyze how the two types of surface textures affect the heat transfer coefficient. A two-dimensional heat transfer approach was proposed to determine the local values of the heat transfer coefficient. The inverse problem for the heated wall was solved using a semi-analytical method based on the Trefftz functions. The results are presented as relationships between the heat transfer coefficient and the distance along the minichannel length and as boiling curves. The experimental data obtained for the two types of enhanced heated surfaces was compared with the results recorded for the smooth heated surface. The highest local values of the heat transfer coefficient were reported in the saturated boiling region for the plate with the type 1 texture produced by vibration-assisted laser machining.

Influence of the Surface Enhancement on the Flow Boiling Heat Transfer in a Minichannel

ABSTRACT This paper presents results concerning flow boiling heat transfer in three parallel vertically oriented and asymmetrically heated rectangular minichannels. Each minichannel was 1.7 mm deep, 16 mm wide, and 180 mm long. The heated element for Fluorinert FC-72 flowing in the minichannels was a thin foil. Infrared thermography was used to determine changes in the temperature on the outer smooth side of the foil. Two-phase flow patterns were observed through a glass pane. The heated surfaces in contact with the fluid in the minichannels differed in roughness. In one minichannel the surface was smooth. In the other two, the surface was enhanced. Two types of surface enhancement were analyzed: a surface with unevenly distributed minicavities and a surface coated with metallic powder applied by soldering. This paper analyzes the effects of the microstructured heated surface on the heat transfer coefficient. The results are presented as: relationships between the heat transfer coefficient and the vapor quality, boiling curves and two-phase flow images. The experimental data obtained for the two types of enhanced surfaces was compared with the results recorded for the smooth surface. The highest local values of the heat transfer coefficient were reported for the enhanced foil with minicavities.

Impact of different thickness of the smooth heated surface on flow boiling heat transfer

This paper presents a comparison of the performance of three smooth heated surfaces with different thicknesses. Analysis was carried out on an experimental setup for flow boiling heat transfer. The most important element of the setup was the test section with a rectangular minichannel, 1.7 mm deep, 16 mm wide and 180 mm long, oriented vertically. The heated element for the FC-72 Fluorinert flowing in the minichannel was designated as a Haynes-230 alloy plate (0.10 mm and 0.45 mm thick) or a Hastelloy X alloy plate (0.65 mm thick). Infrared thermography was used to measure the temperature of the outer plate surface. The local values of the heat transfer coefficient for stationary state conditions were calculated using a simple one-dimensional method. The experimental results were presented as the relationship between the heat transfer coefficients in the subcooled boiling region and the distance along the minichannel length and boiling curves. The highest local heat transfer coefficients were recorded for the surface of 0.10 mm thick heated plate at the outlet and 0.45 mm thick plate at the minichannel inlet. All boiling curves were typical in shape.

The heat transfer coefficient determination with the use of the Beck-Trefftz method in flow boiling in a minichannel

In this paper, the solution of the two-dimensional inverse heat transfer problem with the use of the Beck method coupled with the Trefftz method is proposed. This method was applied for solving an inverse heat conduction problem. The aim of the calculation was to determine the boiling heat transfer coefficient on the basis of temperature measurements taken by infrared thermography. The experimental data of flow boiling heat transfer in a single vertical minichannel of 1.7 mm depth, heated asymmetrically, were used in calculations. The heating element for two refrigerants (FC-72 and HFE-7100, 3M) flowing in the minichannel was the plate enhanced on the side contacting with the fluid. The analysis of the results was performed on the basis of experimental series obtained for the same heat flux and two different mass flow velocities. The results were presented as infrared thermographs, heated wall temperature and heat transfer coefficient as a function of the distance from the minichannel inlet. The results was discussed for the subcooled and saturated boiling regions separately.