Pool boiling heat transfer for surfaces with microchannels of variable depth

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.


Introduction
The paper deals with experimental investigations of boiling heat transfer on a system of parallel horizontal channels. This structures can be applied for cooling miniature integrated devices, such as microprocessors, by a direct or indirect method (as a thermosyphon or tube evaporator), substituting forced convection (traditional fan). An overview of the surfaces with microchannels and the heat transfer coefficients obtained are presented in Table 1.  [2] copper micro-channels 0.2 -0.4 mm wide and 0.100 -0.400 mm deep water 269 kW/m 2 K (channel width 0.375 mm, depth 0.4 mm) Jaikumar and Kandlikar [3] microchannels 300 m, 500 m and 762 m wide, three coating configurations: sintered-throughout, sintered-fin-tops, sintered-channels. water about 60 kW/m 2 K for the channel width 30 m, depths 100 m Walunj and Sathyabhama [15] rectangular, parabolic and stepped microchannels on 10-mm diameter copper rod; channel widths: 250-800 μm; depth: 500 μm.
water about 16 kW/m 2 K The objective of this article is comparison of surfaces with variable and constant depth microchannels with respect to boiling heat transfer enhancement, taking into account microchannels of 0.4 -0.5 mm in wide.

Experimental setup
The diagram of the measurement stand for the determination of boiling curves is presented in Figure 1.
The main stand module ( Fig. 2) consists of a vessel with four flat glass walls (2), filled with working fluid, and placed over the investigated sample (3). The sample was soldered to a 170 mm long cylindrical copper bar of 45 mm diameter. The cylinder diameter corresponded to the diagonal of the sample base. A 1000 W electric cartridge heater of 19 mm diameter and 130 mm long was installed into the bar. Before assembly the heater was coated with a special thermal paste to eliminate air spaces and decrease thermal resistance between the heater and cylinder material.  The samples with microchannels were made of copper and had parallel grooves with a constant pitch, made with an end mill (CNC machining process). The test section with a 27 x 27 mm 2 boiling region consisted of a 32 x 32 mm 2 base square copper sample.

Results
Two types of surfaces (MC and MCV) were examined to study the influence of the kind of extended surface on nucleate boiling heat transfer performance for two different liquids (water, ethanol).
The effect of the microchannel depth and width for water is shown in Fig. 6. The best results, with low and medium heat fluxes (q < 350 kW/m 2 ) were obtained for microchannels with variable depth (MCV). For higher heat fluxes microchannels with constant depth (MC) shows about 10% higher heat transfer coefficients than surface MCV. Boiling heat transfer enhancement for MCV related to plain surface (α/α ps) is about 2 at heat fluxes 100 -350 kW/m 2 .
With ethanol boiling, the performance of microchannels has proved especially good for the MC surface (Fig. 7). Boiling heat transfer enhancement related to plain surface (α/α ps) is 2 -1.5 in the range q = 100 -300 kW/m 2 . Compared with MC surface, MCV surface gave lower heat transfer coefficients especially for higher heat fluxes (q > 300 kW/m 2 ). The mechanism of bubble nucleation, growing and departing is shown in Figs 8 and 9. Vapor bubble generated in microchannel spaces between neighboring microfins in the corner at the microchannel base. It grew and moved towards the microfin tip. The second stage of growth was observed when the bubble adhered to the top of the microfin. Increasing volume of the bubble increased buoyancy force. The growing bubble departed from the microfin top.
For microchannels of variable depth predicted boiling mechanism may be related to macroconvection with separate liquid-vapor pathways as well as with sustion-evaporation mode. Figure 10 shows predicted boiling mechanism for microchannels with changing channels' depth. Small depth channels will contribute to the onset of nucleate boiling (ONB) at low superheating, but at medium heat flux heat transfer coefficient decreases after dry-out heat flux (DHF) is reached. Microchannels with greater depth improve the pool boiling performance at higher heat flux they do not only provide an aditional surface for boiling heat transfer and nucleation, but they can prevent large bubbles from coalescing into a vapor blanket. This kind of vapor layer could cover the whole MC surface and ultimately dry out. Deep microchannels can eliminate this phenomenon, so that the critical heat flux (CHF) will have higher values.

Conclusion
The following conclusions can be drawn from the experiments:  When boiling water on microchannels with constant (MC) and variable depth (MCV), similar heat transfer coefficients (HTC) were obtained. During the boiling of ethanol, higher HTC values occurred at the highest heat fluxes.  Visualization studies allowed the recognition of a two-stage bubble growing mechanism: initially at the bottom of the microchannel, and later at the top of the microfins that limit the microchannel.  More exact conclusions can be drawn after testing a larger number of samples with MC and MCV surfaces in the range of 0.2 mm and 0.6 mm mm.