Magdalena Piasecka

Hysteresis Phenomena at the Onset of Subcooled Nucleate Flow Boiling in Microchannels

In this paper, attempts were made to experimentally investigate the boiling incipience in a narrow rectangular vertical channel of 1 mm depth with an external 40 mm wide wall heated uniformly and others assumed quasiadiabatic. The “boiling front” location was determined from the temperature distribution of the heated wall obtained from liquid crystal thermography. Boiling incipience occurs when a considerable rise in the wall temperature above the saturation temperature takes place. Thus, boiling incipience is accompanied by “nucleation hysteresis.” The impact of various factors on the boiling incipience in microchannels, such as pressure, the inlet liquid subcooling, and flow velocity, were investigated.

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.

Pool boiling on surfaces with mini-fins and micro-cavities

The experimental studies presented here focused on pool boiling heat transfer on mini-fin arrays, mini-fins with perforated covering and surfaces with micro-cavities. The experiments were carried out for water and fluorinert FC-72 at atmospheric pressure. Mini-fins of 0.5 and 1 mm in height were uniformly spaced on the base surface. The copper foil with holes of 0.1 mm in diameter (pitch 0.2/0.4 mm), sintered with the fin tips, formed a system of connected perpendicular and horizontal tunnels. The micro-cavities were obtained through spark erosion. The maximal depth of the craters of these cavities was 15 – 30 μm and depended on the parameters of the branding-pen settings. At medium and small heat fluxes, structures with mini-fins showed the best boiling heat transfer performance both for water and FC-72. At medium and high heat fluxes (above 70 kW/m2 for water and 25 kW/m2 for FC-72), surfaces with mini-fins without porous covering and micro-cavities produced the highest heat transfer coefficients. The surfaces obtained with spark erosion require a proper selection of geometrical parameters for particular liquids – smaller diameters of cavities are suitable for liquids with lower surface tension (FC-72).

Flow Boiling Heat Transfer in A Minichannel with Enhanced Heating Surface

This paper presents the results of flow boiling in a 1.0-mm-deep minichannel with asymmetrical heating. The heating element for the working fluid (FC-72) is a single-sided enhanced alloy foil made from Haynes-230. Two types of enhanced heating surfaces, prepared by laser texturing and with microrecesses varied in terms of size, were used for investigations. The experimental research focused on the transition from single-phase forced convection to nucleate boiling, that is, the zone of boiling incipience and further development of boiling. Flow structure was observed through a glass pane. Owing to the liquid crystal layer placed on the opposite side of the enhanced foil surface, it was possible to observe the onset of flow boiling (as a “boiling front”) and to measure temperature distribution on the heating wall through another glass pane. The objective of the study is to determine void fractions for increasing heat fluxes supplied to the heating surface. The flow structure photos were processed in Corel graphics software and binarized. The analysis of phase volumes was developed in Techsystem Globe and NIS-Elements Advanced Research software. The results of experiments with both types of enhanced heating foil were compared.

Laser Texturing, Spark Erosion and Sanding of the Surfaces and their Practical Applications in Heat Exchange Devices

The paper describes selected passive methods of modifying properties and structures of metal surfaces. The following surface processes with thermal treatment have been used: laser surface texturing, electromachining (spark erosion) and mechanical process sand blasting. Practical aspects of the use of produced surfaces in two types of heat transfer devices have been underlined. The first one consists of the heat exchanger with a minichannel furnished with enhanced heating surface. The second one includes prototype solar collectors with the developed surface of the absorbers pipes or smooth pipes covered with developed absorber plate. Modified developed metal surfaces obtained by selected passive methods reach more effective heat transfer in comparison with smooth surfaces.

Experimental Error Analysis and Heat Polynomial Method Improvement for Boiling Heat Transfer Numerical Calculations in Minichannels

The paper continues the discussion of experimental and numerical investigations of forced convection boiling heat transfer in vertical minichannels covered by two former editions of this conference and our previous papers. Liquid crystal thermography technique was used for measuring the two-dimensional heating surface temperature distribution and boiling front detection. Influence of selected parameters on boiling heat transfer and nucleation hysteresis was observed and discussed. The two-dimensional heat transfer model and the analytic-numerical heat polynomial method were applied to solve the inverse boundary value problem and determine the temperature distributions in the heating foil and protecting glass and the boiling heat transfer coefficient as well. This paper shows how to modify and improve the heat polynomial method if we know the measurement errors and implement them into the numerical procedure. The accuracy of temperature measurements on the heating surface with liquid crystal method was estimated and the analysis of experimental results was given. The functions sought in numerical calculations describe temperature distribution in the protecting glass and the heating foil of the minichannel. They are presented in the form of linear combination of heat polynomials. The adopted boundary conditions and temperature measurements are used to construct error functionals. The latter express the root-mean-square errors, with which computed solutions satisfy relevant boundary conditions. On the basis of functional minimalisation unknown coefficients of linear combinations are determined. The solutions obtained satisfy the differential equations in the exact manner whereas the adopted boundary conditions are met in the approximate fashion. The unknown boiling heat transfer coefficient is the function computed from the boundary condition of the third kind. In the modified method, measurement errors are weights for individual temperature measurements. The more accurate is the measurement, i.e. has a smaller error, the greater is the weight put to it in further calculations. Therefore, it is possible to heighten the accuracy with which glass and foil temperature distributions, determined experimentally, fulfil the assumed equality conditions on the contact surface. Temperature distributions in the glass and the foil, computed on the basis of the modified method, are closer to real values than those obtained with the basic one. Local heat transfer coefficients obtained for two-dimensional boiling heat transfer model with both the basic and the modified heat polynomial methods are also compared.Copyright

Influence of Selected Parameters on Boiling Heat Transfer in Minichannels

The experimental investigations cover heat transfer of refrigerants R 123 and R 11 flowing through vertical minichannels of 40 mm wide rectangular section and depths of 1 mm, 1.5 mm and 2 mm. The heating foil, supplied with controlled direct current, constitutes one of the surfaces of the minichannel. The liquid crystal thermography technique is applied in order to measure the two-dimensional temperature field of the heating surface. The investigations focus on the transition from single-phase forced convection to nucleate boiling, i.e. in the zone of boiling incipience. The present work aims to examine and analyze how the selected parameters (inlet pressure, inlet liquid subcooling, liquid flow velocity) affect nucleate boiling incipience for various geometry (changeable depth) of the minichannel. Furthermore, the investigations are intended to develop a correlation for the calculations of the Nusselt number under the conditions of boiling incipience in the minichannel. The equations are derived as modifications of the already developed ones [Piasecka, 2002; Piasecka and Poniewski, 2003b,c; Piasecka et al., 2004] and as a function of changeable parameters in the experimental investigations.Copyright

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.