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

Single - phase flow and flow boiling of water in rectangular metallic microchannels

This thesis was submitted for the degree of Doctor of Philosophy and awarded Brunel University London.This experimental research aims at investigating the single-phase flow heat transfer and friction factor, flow boiling heat transfer and pressure drop, and flow visualisation in microchannels using de-ionized water. In the literature, many studies failed to explain the effect of aspect ratio on the single-phase and two-phase flow heat transfer rate and pressure drop. Because the channel aspect ratios and hydraulic diameters were varied together in those studies. Also, there is a discrepancy between past studies and the conventional theory for the flow boiling heat transfer characteristics. Accordingly, the objectives of this research can be listed as follows: (i) modifying the existing experimental facility to perform single-phase and two-phase flow heat transfer and pressure drop and two-phase flow pattern visualization experiments in microchannels, (ii) clarifying the fundamental aspects of flow boiling in micro passages, (iii) investigating the aspect ratio, heat flux, mass flux and vapour quality effects on flow patterns, heat transfer rate and pressure drop in single-phase and two-phase flow, (iv) comparing the obtained results with heat transfer and pressure drop correlations and flow pattern maps available in the literature. Consequently, the pre-existing experimental facility was modified in the current research by changing the pre-heaters, flowmeter and piping in order to achieve the goals of this study. Four copper rectangular microchannels were designed and manufactured. Three microchannel test sections having the same hydraulic diameter and length but different aspect ratios were investigated to reveal the effect of aspect ratio on the single-phase and two-phase flow heat transfer rate and pressure drop. The surface roughness of each microchannel was also examined. It was found that the surface roughnesses of all microchannels are similar. Moreover, an additional microchannel test section was used to examine the effect of heated length on the flow boiling heat transfer coefficient and pressure drop. The single-phase flow results demonstrated that the channel aspect ratio has no influence on the friction factor and heat transfer rate for the tested microchannels and experimental range. In the flow boiling experiments, bubbly, bubbly/slug, slug, churn and annular flow regimes were observed in the tested microchannels. The channel aspect ratio effect was found to be small on the observed flow patterns. The experimental flow patterns were predicted well by the flow pattern map proposed by Galvis and Culham (2012) except for the slug flow regime. The flow pattern maps of Sobierska et al. (2006) and Harirchian and Garimella (2009) reasonably predicted the experimental flow pattern data. The flow boiling heat transfer results showed that the prevailing heat transfer mechanism is nucleate boiling for the low and medium heat flux inputs. On the other hand, the dominant heat transfer mechanism is unclear at the high heat flux inputs while smaller aspect ratio microchannel has better heat transfer performance for low and medium heat flux inputs. However, at high heat flux inputs the channel aspect ratio effect was found to be insignificant on the flow boiling heat transfer coefficient. The experimental flow boiling heat transfer coefficient data were reasonably predicted by the correlations of Sun and Mishima (2009), Li and Wu (2010) and Mahmoud and Karayiannis (2011) from the literature. The flow boiling pressure drop characteristics were also examined in the tested microchannels. Outcome of the experiments consistently indicated a highly linear trend between the increasing flow boiling pressure drop and the heat and mass flux. Also, the flow boiling pressure drop increased with the increase in vapour quality. The effect of channel aspect ratio on the flow boiling pressure drop was also assessed. It was found that when the channel aspect ratio decreased, the flow boiling pressure drop increased. The experimental flow boiling pressure drop data were compared to correlations from the literature. Mishima and Hibiki (1996), Yu et al. (2002) and Zhang et al. (2010) correlations reasonably predicted the experimental flow boiling pressure drop results.Turkish Ministry of Higher Education Council and Marmara Universit

Exergy Analysis of Ferrofluid Nanofluid under Single-Phase and Pool Boiling Conditions

Son yıllarda termal-sıvı uygulamaları artan ısı akısını karşılamak için en sık kullanılan yöntemlerden biri olmaya başlamıştır.Bu uygulamalardan en popüler olanlarından biri sıvıya nano-parçacık karıştırarak ısı transfer hızını arttırmaya çalışmaktır.Teoride kabul gören bu yöntem için farklı araştırma gruplarından farklı sonuçlar gelmekle birlikte kesin bir yargıya henüz tamolarak ulaşıldığı söylenemez. Sıvıya nano-parçacık eklemenin en zorlu yanı, birçok çalışmada belirtildiği gibi nanoparçacıklarınyüzey üzerinde kümelenmeye ve çökelmeye meyilli olması ve bu durumun olması halinde ısı transferine negatifetki yapmasıdır. Bu özelliklerinden ötürü nano-parçacıkların sistem üzerinde kararsız davranış oluşturduğu da bazı çalışmalardarapor edilmiştir. Bu çalışmada ana sıvı olarak suya Fe3O4 nano-parçacıkları eklenen sistemin ekserji analizi yapılmıştır. Buradaen önemli nokta, sistemin manyetik kuvvete maruz bırakılması olup bu sayede çökelme ve kümelenmeye fırsat verilmeyecekolmasıdır. Bu çalışmada literatürden farklı olarak sistemin tek-fazlı akış ve havuz kaynama şartlarındaki verimi ekserjetik verimüzerinden değerlendirilecektir. Sonuçlar saf su, su-Fe3O4 nano-sıvısı ve manyetik kuvvet altındaki su-Fe3O4 nano-sıvısışeklinde sunulup ekserji yıkım oranları karşılaştırılmıştır.In recent years, thermal-liquid applications have become one of the most commonly used methods to meet high heat flux_x000D_ demand. One of the most popular of these applications is to try to enhance the heat transfer rate by mixing nanoparticles into_x000D_ the liquid. Although different results come from different research groups for this method, which is accepted in theory, it cannot_x000D_ be said that a satisfactory conclusion has been reached yet. The most challenging aspect of adding nanoparticles to the liquid_x000D_ is that, as noted in many studies, the nanoparticles tend to cluster and sediment on the surface and, if this happens, have a_x000D_ negative effect on heat transfer. Due to these characteristics, some studies have reported that nanoparticles cause unstable_x000D_ behaviour on the system. In this study, exergy analysis of Fe3O4-water nanofluid has been performed. The most important point_x000D_ here is that the system is exposed to magnetic force, so that clustering and sedimentation is prevented. In this study, unlike the_x000D_ literature, the efficiency of the system will be evaluated on the basis of exergetic efficiency. The results will be presented for_x000D_ pure water, Fe3O4-water nanofluid and Fe3O4-water under magnetic actuation and their exergy destruction rate values will be_x000D_ compared and discussed

FLOW BOILING HEAT TRANSFER IN A RECTANGULAR COPPER MICROCHANNEL

Flow boiling characteristics of de-ionized water were tested experimentally in a rectangular copper single microchannel of 1 mm width, 0.39 mm height and 62 mm length. De-ionized water was supplied to the microchannel at constant inlet temperature (89 degrees C) and constant inlet pressure (115 kPa). The mass flux ranged from 200 to 800 kg/m(2)s and the heat flux from 56 to 865 kW/m(2). The heat transfer rate data are presented as plots of local heat transfer coefficient versus vapour quality and distance along the channel. Flow visualization was also conducted using a high-speed, high-resolution camera. The results indicate that unstable flow boiling occurred starting at boiling incipience for all mass flux values. The local heat transfer coefficient depends on heat flux only at very low heat and mass fluxes. At high mass flux, there is no heat flux effect with little dependence on vapour quality after the entry region. The mass flux effect was more complex

Design and implementation of minichannel evaporator for electronics cooling

The present study elucidates the design and experimentation of a minichannel evaporator in an R134a vapour compression refrigeration system for electronics cooling applications. In the current study, a calculation module was developed to design a minichannel evaporator to keep the surface temperature of the chip below a certain value for reliable operation conditions in electronic cooling applications. In the calculation module, the conventional-scale heat transfer correlation was used to predict the surface temperature of the chip. On the other hand, the conventional-scale and microscale pressure drop correlations were tested to assess the pressure drop in the minichannel evaporator. The proposed calculation module was verified using experimental tests for different heat loads. It was found that the proposed calculation model predicted very well the experimental data of the surface temperature of the chip for all heat input. The calculation module with micro-scale pressure drop correlation predicted well the experimental pressure drop data in the minichannel evaporator for all heat loads. Moreover, the effects of the degree of subcooling, superheating degree and condensation temperature on the surface temperature of the chip and pressure drop in the minichannel evaporator were investigated to determine optimum operating conditions at different cooling capacities using the calculation module. The results showed that the increase in the degree of subcooling enhances the performance of the minichannel evaporator. On the other hand, the lower degree of superheating and condensation temperature yielded better performance for the minichannel evaporator. The feasibility of the results for electronic cooling applications is discussed based on the findings

Demir Bazlı Nano-Sıvının Tek-Faz ve Havuz Kaynama Isı Transferi Şartlarında Ekserji Analizi

Son yıllarda termal-sıvı uygulamaları artan ısı akısını karşılamak için en sık kullanılan yöntemlerden biri olmaya başlamıştır.Bu uygulamalardan en popüler olanlarından biri sıvıya nano-parçacık karıştırarak ısı transfer hızını arttırmaya çalışmaktır.Teoride kabul gören bu yöntem için farklı araştırma gruplarından farklı sonuçlar gelmekle birlikte kesin bir yargıya henüz tamolarak ulaşıldığı söylenemez. Sıvıya nano-parçacık eklemenin en zorlu yanı, birçok çalışmada belirtildiği gibi nanoparçacıklarınyüzey üzerinde kümelenmeye ve çökelmeye meyilli olması ve bu durumun olması halinde ısı transferine negatifetki yapmasıdır. Bu özelliklerinden ötürü nano-parçacıkların sistem üzerinde kararsız davranış oluşturduğu da bazı çalışmalardarapor edilmiştir. Bu çalışmada ana sıvı olarak suya Fe3O4 nano-parçacıkları eklenen sistemin ekserji analizi yapılmıştır. Buradaen önemli nokta, sistemin manyetik kuvvete maruz bırakılması olup bu sayede çökelme ve kümelenmeye fırsat verilmeyecekolmasıdır. Bu çalışmada literatürden farklı olarak sistemin tek-fazlı akış ve havuz kaynama şartlarındaki verimi ekserjetik verimüzerinden değerlendirilecektir. Sonuçlar saf su, su-Fe3O4 nano-sıvısı ve manyetik kuvvet altındaki su-Fe3O4 nano-sıvısışeklinde sunulup ekserji yıkım oranları karşılaştırılmıştır.In recent years, thermal-liquid applications have become one of the most commonly used methods to meet high heat flux_x000D_ demand. One of the most popular of these applications is to try to enhance the heat transfer rate by mixing nanoparticles into_x000D_ the liquid. Although different results come from different research groups for this method, which is accepted in theory, it cannot_x000D_ be said that a satisfactory conclusion has been reached yet. The most challenging aspect of adding nanoparticles to the liquid_x000D_ is that, as noted in many studies, the nanoparticles tend to cluster and sediment on the surface and, if this happens, have a_x000D_ negative effect on heat transfer. Due to these characteristics, some studies have reported that nanoparticles cause unstable_x000D_ behaviour on the system. In this study, exergy analysis of Fe3O4-water nanofluid has been performed. The most important point_x000D_ here is that the system is exposed to magnetic force, so that clustering and sedimentation is prevented. In this study, unlike the_x000D_ literature, the efficiency of the system will be evaluated on the basis of exergetic efficiency. The results will be presented for_x000D_ pure water, Fe3O4-water nanofluid and Fe3O4-water under magnetic actuation and their exergy destruction rate values will be_x000D_ compared and discussed

A review on laminar-to-turbulent transition of nanofluid flows

Nanofluids have emerged as powerful instruments in heat transfer applications due to their improved thermophysical properties. Additionally, many heat transfer equipments are started to be operated within the range of transitional flow regions in the advances in thermal management enhancement techniques. However, up to date, the friction factor and heat transfer coefficient features of nanofluids within the transitional flow regions and the effect of nanoparticle addition into the base fluid on the laminar-to-turbulent transition characteristics are still not understood clearly with contradictory published results. At this point, this paper comprehensively reviews the studies dealing with the nanofluid flow within the transitional flow regions for internal flow applications. After the presentation of applications of nanofluid flow in the transitional flow regions, the nanofluid properties such as nanoparticle type and concentration and base fluid type in the reviewed studies are given in detail. The pressure drop and heat transfer features of nanofluid flow within the transitional flow regions are distinctly identified and discussed for internal flows. The effect of the nanoparticle addition into the liquid on the transition onset is discussed with results from different research groups. A complete evaluation, challenges and further studies are proposed based on available results in the literature