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The work focused on the investigations and comparison of pool boiling heat transfer on the specimens with in 5 mm deep and 1 mm wide parallel mini-channels to the specimens with additional porous material (copper foam or mesh) between the fins. The experiments were carried out with boiling FC-72 at atmospheric pressure. Many works show that enhanced surfaces with open minichannels improve heat transfer performance substantially. The use of additional porous material led to the highest heat transfer coefficients in the range of 100 to 230 kW/m2. The coefficients obtained were 15% higher and the superheat was 15% lower compared to the plain smooth minichannel without porous material.

The paper presents investigations into pool boiling heat transfer for open minichannel surfaces. The experiments were carried out with saturated ethanol at atmospheric pressure. Parallel microchannels fabricated by machining were about 0.2, 0.3 and 0.4 wide and 0.2 to 0.5 mm deep. The measurements were performed with increasing heat flux and variable geometric parameters of the minichannels. The image acquisition speed was 493 fps (at resolution 400 x 300 pixels with Photonfocus PHOT MV-D1024-160-CL camera) and an EX-FH20 (Casio) camera was used to record the images of the entire surface of the specimen. The analysis of boiling curves for the tested surfaces does not give an unambiguous response to the influence of geometrical parameters, i.e. the height of the microchannels on the heat transfer process.

The article describes the experimental investigation of pool boiling heat transfer on minichannels with filling in the form of porous structure (copper foam). The results were compared with the data for a smooth surface and the minichannels without additional fillings. Tests were carried out for the boiling liquid Novec-649. Surfaces partially filled with porous structure were formed by inserting pieces of copper foam into the minichannels of 5 mm in depth and 1 mm width. Minichannels completely filled with copper foam formed the surface of MCC-F. The measurements were made with an increase in heat flux. The heat transfer coefficient obtained was four times higher than for the smooth surface. Additional foam fillings increased the heat transfer coefficient and reduced superheat for heat fluxes less than 100 kW/m2. Visualisation was made using a high-speed camera which allowed to determine the diameters of the growing bubble.

The paper presents investigations into pool boiling heat transfer for open microchannel surfaces. The experiments were carried out with saturated FC-72 at atmospheric pressure. Parallel microchannels fabricated by machining were about 0.2 to 0.4 mm wide and 0.2 to 0.5 mm deep. Analyzed surfaces with microchannels allowed to obtain heat transfer coefficients within the range of 6.1 – 9.8 kW/m2K, which in relation to the flat surface gives a 3 – 5 - fold increase in HTC. One of the reasons for the increase in the heat transfer coefficient when increasing the heat flux was the growing number of active nucleation sites at the bottom of microchannels and its side surfaces.

Experimental investigations of pool boiling heat transfer on microchannels of variable depth were conducted. The experiments were carried out for water and ethanol at atmospheric pressure. Microchannels of variable depth from 0.2 to 2.8 mm and width 0.5 mm were uniformly spaced on base surface with pitch of 1 mm. The comparison of heat transfer coefficients for surfaces with variable and constant depth of microchannels was made. At the low and medium heat fluxes structures with constant microchannel depth showed the best boiling heat transfer performance. EX-FH20 (Casio) camera was used to record the images of the entire surface of the specimen. The bubble growth mechanism on the enhanced surface was different from that of plain surface. Visualization investigations were aimed at identifying nucleation sites and determining the bubble growth cycle. Vapor bubbles generate in microchannel spaces, from where they move towards the fin tips, then grow and depart.

Saturated pool boiling from copper microchannel surfaces was examined using Novec-649 and FC-72 as working fluids. Parallel microchannels fabricated by machining were about 0.2 mm wide, and 0.2 to 0.4 mm deep and spaced every 0.4 mm. The experiments were carried out at atmospheric pressure. The images of the growing and departing bubbles for the entire surface of the specimen were taken with a Photonfocus PHOT MV-D1024-160-CL camera and an EX-FH20 (Casio) camera. At low superheats, the bubbles departing from enhanced surfaces were spherical in shape and did not coalesce. At higher superheats, the bubbles interacted with each other intensively, forming flattened spherical, funnel-shaped bubbles at the bottom. The visualization study aimed at identifying nucleation sites of the departing bubbles and determining their diameters and frequency at various superheats.

The boiling curves and heat transfer coefficients between the heating surface and fluid were investigated in the paper. Copper samples with horizontal microchannels of rectangular cross-section, variable depth and width were the objects of the study. The following geometrical parameters have been used: microchannel width 0.2; 0.3 and 0.4 mm, depth between 0.2 and 0.5 mm (change every 0.1 mm). Boiling refrigerant was Novec-649 (GWP = 1), and the experiment was performed at atmospheric pressure. Geometrical parameters impact, within a given range of heat flux 3 – 130 kW/m2, on the heat transfer process was determined.

The paper describes results of analysis of pool boiling heat transfer on enhanced surfaces. Two types of structural surfaces were used: open microchannel surfaces consisting of a system of parallel micro-channels 0.3 mm wide, from 0.2 to 0.5 mm deep and with a pitch of 0.6 mm, and plain micro-fins 0.5 mm in height, uniformly spaced on the base surface with a spacing from 0.6 to1.5 mm. Pool boiling data at atmospheric pressure were obtained for saturated water, ethanol and FC-72. The effects of micro-channel/micro-fin dimensions on heat transfer coefficient in nucleate pool boiling were examined. Substantial enhancement of heat transfer coefficient was observed.

The paper presents experimental investigations into pool boiling heat transfer for open microchannel surfaces. Parallel microchannels fabricated by machining were about 0.3 mm wide, and 0.2 to 0.5 mm deep and spaced every 0.1 mm. The experiments were carried out for ethanol, and FC-72 at atmospheric pressure. The image acquisition speed was 493 fps (at resolution 400 × 300 pixels with Photonfocus PHOT MV-D1024-160-CL camera). Visualization investigations aimed to identify nucleation sites and flow patterns and to determine the bubble departure diameter and frequency at various superheats. The primary factor in the increase of heat transfer coefficient at increasing heat flux was a growing number of active pores and increased departure frequency. Heat transfer coefficients obtained in this study were noticeably higher than those from a smooth surface.

The paper presents visualization investigations into pool boiling heat transfer for open microchannel surfaces. The experiments were carried out with saturated water, ethanol, FC-72 and Novec-649 at atmospheric pressure. Parallel microchannels fabricated by machining copper sample were about 0.2 to 0.5 mm wide and 0.2 to 0.5 mm deep. The diameter of departing bubble was calculated for the microchannel surface on the basis of buoyancy force and surface tension force balance. The visualization carried out was aimed at determining the diameters of the departing bubbles at various heat fluxes for four working fluids.

This study focuses on the effect of channel depth on the heat transfer coefficient during nucleate pool boiling. Experimental studies were performed for saturated deionized water, and Novec-649 as working fluids at atmospheric pressure. Copper surfaces were modified to form microchannels with different geometrical properties. The microchannels were from 0.2 mm to 0.4 mm deep, 0.3 mm wide and spaced every 0.1 mm. The experiment was conducted for increasing heat flux up to the critical heat flux point. The surface modification provided an appreciably higher heat transfer coefficient compared to the smooth surface for all boiling liquids. The maximum heat transfer coefficient obtained exceeded 60 kW/m2K.

The paper presents visualization investigations into pool boiling heat transfer for open minichannel surfaces. The experiments were carried out wih saturated water at atmospheric pressure. Parallel microchannels fabricated by machining were about 0.3 mm wide and 0.2 to 0.4 mm deep. High-speed videos were used as an aid to understanding the heat transfer mechanism. The visualization study aimed at identifying nucleation sites of the departing bubbles and determining their diameters and frequency at various superheats.

The paper presents experimental studies of nucleate boiling heat transfer from a system of connected horizontal and vertical subsurface tunnels. The experiments were carried out for water at atmospheric pressure. The tunnel external covers were manufactured out of perforated copper foil (holes diameter 0.3 mm), sintered with the mini-fins, formed on the vertical side of the 10 mm high rectangular fins and horizontal inter-fin surface. The image acquisition speed was 493 fps (at resolution 400 × 300 pixels with Photonfocus PHOT MV-D1024-160-CL camera). Visualization investigations aimed to identify nucleation sites and flow patterns and to determine the bubble departure diameter and frequency at various superheats for vertical tunnels. At low superheat vapor bubbles are generated nearly exclusively by the vertical tunnel. At medium values of superheat, pores of the horizontal tunnel activate.

The heat transfer measurements were conducted during pool boiling of water on surfaces with microchannels. Parallel grooves were made on a copper surface with widths ranging from 0.2 mm to 0.5 mm at intervals of 0.1 mm. The inclination angle of the grooves to the horizontal was set at 30° and 60°, and the depth of the microchannel grooves was 0.3 mm. The achieved heat flux ranged from 25 kW/m² to 1730 kW/m², and the heat transfer coefficients ranged from 12 kW/(m²K) to 475 kW/(m²K). The influence of geometric parameters such as width, inclination angle of the microchannel, surface extension, and Bond number on heat exchange efficiency was examined. A nearly sixfold increase in a (heat transfer coefficient) and a twofold increase in critical heat flux were observed compared to a smooth surface. [ABSTRACT FROM AUTHOR]

In this paper, pool boiling of ethanol at atmospheric pressure was analyzed. The enhanced surfaces were made of copper, on which grooves with a depth ranging from 0.2 to 0.5 mm were milled in parallel. The widths of the microchannels and the distances between them were 0.2 mm, 0.3 mm and 0.4 mm, respectively. The highest heat transfer coefficient, 90.3 kW/m2K, was obtained for the surface with a microchannel depth of 0.5 mm and a width of 0.2 mm. The maximum heat flux was 1035 kW/m2. For the analyzed surfaces, the maximum heat flux increase of two and a half times was obtained, while the heat transfer coefficient increased three-fold in relation to the smooth surface. In the given range of heat flux 21.2–1035 kW/m2, the impact of geometric parameters on the heat transfer process was presented. The diameters of the departing bubbles were determined experimentally with the use of a high-speed camera. A simplified model was proposed to determine the diameter of the departure bubble for the studied surfaces. [ABSTRACT FROM AUTHOR]

In the paper, the results of tests on pool boiling heat transfer on open microchannel surfaces were discussed. Nucleate pool boiling from copper microchannel surfaces was examined using ethanol as a working fluid. Parallel microchannels fabricated by machining were about 0.2 to 0.4 mm wide and 0.2 to 0.5 mm deep. In the mathematical model the heat transfer process was in steady state. The heat transfer in copper block was assumed axisymmetric while the specimen temperature distribution was not. The temperature of the copper block and the specimen were assumed to satisfy Laplace's equations with adequate boundary conditions. The problem was solved by the Trefftz method with two sets of Trefftz functions. Graphs were used to represent the copper block and the specimen temperature distributions, local values of heat transfer coefficients as a function of the heat flux and fin temperature distribution. Uncertainty analysis was presented. [ABSTRACT FROM AUTHOR]

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