Pressure drop across micro-pin heat sinks under boiling conditions

Two-phase pressure drop was studied in four different micro pin fin heat sinks. Micro pin fin heat sinks used in the current studies were operated under boiling conditions using water and R-123 as working fluids. It was observed that once boiling was initiated severe temperature fluctuations and flow oscillations were recorded for three of the micro pin fin heat sinks, which was characterized as unstable boiling. Pressure drop signals were presented just before and after the unstable boiling conditions. Flow images and FFT (fast Fourier Transform) profiles of pressure signals were used to explain experimental results and unstable nature in flow boiling observed in the three of the devices. Stable boiling conditions where the temperature and pressure drop had a steady and stable profile could be only obtained from one micro pin fin heat sink at high mass velocities. The two-phase pressure drop in this hydrofoil-based micro pin fin heat sink has been investigated using R-123 as the working fluid. Two-phase frictional multipliers have been obtained over mass fluxes from 976 to 2349 kg/m2. It has been found that the two-phase frictional multiplier is strongly dependent on flow pattern. The theoretical prediction using Martinelli parameter based on the laminar fluid and laminar gas flow represented the experimental data fairly well for the spray-annular flow. For the bubbly and wavy-intermittent flow, however, large deviations from the experimental data were recorded. The Martinelli parameter was used successfully to determine the flow patterns, which were bubbly, wavy-intermittent, and spray-annular flow in the current study

Experimental study on single phase flow and boiling heat transfer in microchannels at high flow rates

With the increasing speed and decreasing size of microprocessors and microchips the sizes of their heat sinks are continuously shrinking from mini size to micro size. The most practical and extensively used micro heat sinks are plain microchannels. They find application in many areas. The proposed study aims at filling the gap in single-phase fluid flow and boiling heat transfer in microchannels at high mass velocities in the literature. This thesis presents a two-part study. In both part, fluid flow was investigated over a broad range of mass velocity in a microchannel with different inner diameters. De-ionized water was used as working fluid, and the test section was heated by Joule heating. The wall temperatures and pressure drops were measured and processed to obtain heat transfer coefficients, Nusselt numbers, and friction factors as output. It was found that existing theory for developing flow in conventional scale could fairly predict experimental data on developing flows in microscale for both laminar and turbulent conditions. In the second part of the study, boiling heat transfer experiments have been carried out for the same microchannel configurations. Heat transfer coefficients and qualities were deduced from local temperature measurements. It was found that high heat removal rates can be achieved at high flow rates under subcooled boiling conditions. It was observed that heat transfer coefficients increase with mass velocity, whereas they decrease with local quality and heat flux. Moreover, experimental heat flux data were compared with partial boiling correlations and fully developed correlations

Experimental study on single phase flow in microchannels at high mass flow rates

With the increasing speed and decreasing size of current microprocessors and microchips the dimensions of their heat sinks are continuously shrinking from mini size to micro size. The most extensively used and practical micro heat sinks are plain microchannels which find applications in many areas besides electronics cooling such as in microreactors, fuel cells, drug delivery, micropropulsion and automotive industry. Because of their widespread usage, they attracted the attention of many researchers, which gave rise to many studies on single-phase as well as on flow boiling. The proposed study aims at filling the gap in heat and fluid flow in microchannels at high mass velocities in the literature. For this purpose single-phase fluid (de-ionized water) flow was investigated over a broad range of mass velocity (1300 kg/m(2)s-7200 kg/m(2)s) in a microtube with an inner diameter of similar to 250 mu m. Besides comparing the experimental results in fully developed flow to the theory, the focus of this study is on thermally developing flows. Wall temperatures and pressure drops were measured and processed to obtain heat transfer coefficients, Nusselt numbers and friction factors. It was found that the existing theory about developing flows could fairly predict experimental data on developing flows in microscale for both laminar and turbulent conditions

Thermally developing single-phase flows in microtubes

The pressure drop and heat transfer due to the flow of de-ionized water at high mass fluxes in microtubes of ~ 254 ;m and ~ 685 ;m inner diameters is investigated in the laminar, transition and the turbulent flow regimes. The flow is hydrodynamically fully developed and thermally developing. The experimental friction factors and heat transfer coefficients are respectively predicted to within ±20 % and ±30 % by existing open literature correlations. Higher single phase heat transfer coefficients were obtained with increasing mass fluxes, which is motivating to operate at high mass fluxes and under thermally developing flow conditions. The transition to turbulent flow and friction factors for both laminar and turbulent conditions were found to be in agreement with existing theory. A reasonable agreement was present between experimental results and theoretical predictions recommended for convective heat transfer in thermally developing flows

Boiling heat transfer in microtubes at high flow rates

Boiling heat transfer is an important heat removal mechanism for cooling applications in micro scale and finds many applications. Many studies were conducted to shed light on boiling heat transfer in microchannels. They were concentrated on saturation boiling at low mass fluxes (G<1000 kg/m(2)s). In the current study, the emphasis is on high mass fluxes unlike in the literature. Thus, the current study addresses to the lack of information about boiling heat transfer at high flow rates and aims at presenting necessary experimental data. For this, an experimental study was conducted at high flow rates in microtubes. The microtube sizes were used as 250 micrometers. Deionized water was used as working fluid, and the test section was heated by Joule heating. Mass flux was changed from 1000 kg/m(2)s to 7500 kg/m(2)s, and heat transfer coefficients and qualities were deduced from local temperature measurements. The effects of mass velocity and quality on boiling heat transfer coefficient were investigated

Parametric study on the effect of endwalls on fluid flow in micro pin-fins

The flow across micro pin fins can be considered as cross flow across intermediate size (based on the height-diameter ratio, H/D ratio changes from 1/2 to 5) fins, whose thermal and hydraulic performances are different from long conventional tubes due to the effects of endwalls on the both thermal and velocity boundary layers as well as low flow rates. The endwalls effects imposed by the microchannel having micro pin fins have not been extensively discussed in the literature since mostly long fins have been used at high flow rates, where such effects were mostly negligible. This article intends to address to the lack of information on endwalls effects on fluid flow in micro pin fin heat sinks having intermediate size pin fins under broad working conditions. Water was used as working fluid in the following Reynolds number ranges (1<Re<1000). Heat flux was changed from 30W/cm(2) to 60W/cm(2)

High mass flux flow boiling and critical heat flux in microscale

This study explores critical heat flux in flow boiling of de-ionized water at high mass fluxes in ~249 µm and ~494 µm inner diameter tubes, which are heated along short heated lengths (2-10 mm). Ultra high heat flux cooling (>10000 W/cm2) is achieved in micro scale at high mass fluxes (>10000 kg/m2s). Input heating powers were measured and processed to obtain critical heat fluxes (CHF). It was shown that high heat removal rates more than 30000 W/cm2 were attained with a reasonable combination of working conditions (short heated length-high mass flux-small hydraulic diameter), which can be proposed as a promising solution method for possible new emerging applications such as nano-scale plasmonic applications, near field radiative energy exchange between objects, and nano thermophotovoltaics. Experimental results were compared with the existing CHF prediction methods recommended for low quality flow boiling, and a new correlation for predicting CHF was proposed

Parametric study on the effect of end walls on heat transfer and fluid flow across a micro pin-fin

Micro heat sinks have a broad applicability in many fields such as aerospace applications, micro turbine cooling, micro reactors, electronics cooling, and micro biological applications. Among different types of micro heat sinks, those with micro pin-fins are becoming popular due to their enhanced heat removal performance. However, relevant experimental data in current literature is still scarce to adequately explain their differences from their macro size counter parts. In previous studies in literature, it was shown that thermal and hydrodynamic characteristics of micro pin-fin heat sinks are strongly affected by height over diameter (H/D) ratio of pin-fins. To address this lack of information, the objective of this work is to show how velocity boundary layer around pin-fins and consequently, the thermal and hydrodynamic characteristics are affected when H/D ratio and local Reynolds number (Re) vary. To investigate end wall effects, a small portion of a typical micro pin-fin heat sink is modeled. This portion is represented by a simplified model, which consists of a single pin-fin positioned in a rectangular micro channel. This approach simplified the micro heat sink, which is simulated for only half of it by using a symmetry plane. Moreover, the transverse channel walls are kept as close as the minimum distance (1.5D) between pin-fins available in the literature. In this paper, the pin-fin height over diameter ratio, H/D, varies from 0.5 to 5, while Reynolds number and heat flux provided from the fluid interacting surfaces of the micro pin-fin are in the range of 20≤Re≤150 and 100≤qin (W/cm2)≤500, respectively. In this research, micro pin-fin heat sinks are three dimensionally modeled on a one-to-one scale with the use of commercially available software COMSOL Multiphysics 3.5a. Full and temperature dependent Navier-Stokes equations subjected to compressibility and energy equations are solved under steady state conditions. In order to validate the use of numerical models, simulation results are compared against theoretical predictions. The numerical results and theoretical predictions show a good agreement. After this validation, parametric analysis is performed using the three dimensional model developed with COMSOL Multiphysics 3.5a. The end wall effects are quantified, and this amount decreases with Re and H/D. It is revealed that end walls play an important role on the total fluidic force acting on the micro pin-fin and on the heat transfer coefficients. Moreover, the trends in the amount of end walls effects, the ratio of viscous over total forces on the pin-fin, friction factors, and Nusselt numbers change at various critical Reynolds numbers. It is also demonstrated that increasing H/D ratio leads to a less stable flow, higher fluidic forces on the micro pin-fin with an increased partial role of viscous forces relative to pressure forces, smaller friction factors, and higher heat transfer coefficients. There are maxima and minima in Nusselt number profiles for different H/D ratios. It is found that increasing Re has a positive role in Nusselt numbers, as well as a parallel effect with H/D on fluidic forces on micro pin-fin, friction factors, and heat transfer coefficients. Different than the effect of H/D, Re decreases the partial role of viscous forces relative to pressure forces

Low mass quality flow boiling in microtubes at high mass fluxes

In this article, an experimental study on boiling heat transfer and fluid flow in microtubes at high mass fluxes is presented. De-ionized water flow was investigated over a broad range of mass flux (1000 kg/m2s-7500 kg/m2s) in microtubes with inner diameters of ~ 250 micrometers and ~ 685 micrometers. The reason for using two different capillary diameters was to investigate the size effect on flow boiling. De-ionized water was used as working fluid, and the test section was heated by Joule heating. Heat transfer coefficients and qualities were deduced from local temperature measurements. It was found that high heat removal rates could be achieved at high flow rates under subcooled boiling conditions. It was also observed that heat transfer coefficients increased with mass flux, whereas they decreased with local quality and heat flux. Moreover, experimental heat flux data were compared with partial boiling correlations and fully developed correlations. It was observed that at low wall superheat values there was only a small inconsistency between the experimental data and the conventional partial boiling prediction method of Bergles, while the subcooled and low quality fully developed boiling heat transfer correlation of Kandlikar could fairly predict experimental results at high wall superheat values