Parallel Rectangular Channels Research Articles

EXPERIMENTAL STUDY ON TWO-PHASE NONLINEAR OSCILLATION BEHAVIOR OF MINIATURIZED GRAVITATIONAL HEAT PIPE

An experimental study on oscillation behavior of vapor-liquid two-phase flow in an ethanol-filled aluminum miniaturized gravitational heat pipe with parallel rectangular channels is carried out. The nonlinear oscillation behavior and oscillation characteristics of two-phase flow in the oscillation state are qualitatively analyzed. In addition, the nonlinear oscillation characteristic of the wall temperature is revealed. Furthermore, the influence of heat load on temperature oscillation characteristics is further analyzed. The research shows that the independent oscillation in a single channel characterized by the random formation and oscillation of liquid plug in a single channel is the typical oscillation behavior of two-phase flow under medium heat load (<i>Q</i> = 28 W). In addition, the interaction on oscillation of liquid plugs between adjacent channels is small. For high heat load (<i>Q</i> = 45 W), the vapor pressure difference at both ends of the liquid plug has a more important role in promoting the liquid plug, the superposition of the liquid plug oscillation in single channel and the oscillation between adjacent channels forms the interference oscillation in adjacent channels. For interference oscillation in adjacent channels, under coupling erosion of the liquid plug in a single channel and the liquid plugs in adjacent channels, with increase of heat load (<i>Q</i> increases from 38 W to 45 W), the dominant frequency of oscillation for independent oscillation in a single channel increases (from 2.5 Hz to 4.2 Hz), and the larger driving force caused by heat load increase enhances the oscillation of liquid plug in a single channel and weakens the oscillation of liquid plug in different channels for interference oscillation in adjacent channels.

Experimental investigation of heat transfer to carbon dioxide in parallel rectangular channels under supersonic mainstream

Active cooling using carbon dioxide(CO2) is a promising method for supersonic vehicles' combustion chamber, but few studies have considered CO2 heat transfer under supersonic mainstream conditions. In this paper, the double-wall heat transfer effect of CO2 under supersonic mainstream conditions is investigated by using the direct-connection experimental platform. The double-wall consists of an inner liner and a cooling jacket with parallel rectangular channels. The inlet CO2 parameter conditions range for T (Temperature) = 263.3 ∼ 272.5 K, m (Mass flow rate) = 50 ∼ 205.3 g/s, and P (Pressure) = 3.87 ∼ 8.88 MPa. It has been found that the magnitude of CO2 mass flow rate has a very significant effect on the effectiveness of thermal protection. The CO2 active cooling provides effective thermal protection for the inner liner at inlet parameter m = 199.1 ∼ 205.3 g/s, P = 8.69 ∼ 8.88 MPa, and T = 263.3 ∼ 266.8 K. Moreover, the heat transfer reaches thermal equilibrium quickly. However, it is more effective in thermally protecting the inner liner near the CO2 inlet than near the CO2 outlet at the inlet parameters of m = 50 ∼ 88.6 g/s, P = 3.87 ∼ 7.74 MPa, and T = 263.3 ∼ 272.5 K. In addition, there may be a risk of structural burnout near the CO2 outlet.

Flow condensation inside a multiport mini channel and a rectangular mini channel with pin fin array

In this paper, an experimental study of flow condensation was carried out of R134a in a multiport mini channel and a mini channel with pin fin array. The former comprised 20 parallel rectangular channels with an equivalent diameter of 0.64 mm. The latter is a narrow rectangular mini channel containing 10 rows of diamond pin fins with a staggered configuration, and height and longitudinal/transverse pitch of 0.5 mm and 2 mm respectively. To acquire the local value of heat flux and heat transfer coefficient, air jet impingement cooling devices were adopted to condense the vapor of refrigerants. The effects of vapor quality, mass flux, heat flux, and saturation pressure on flow condensation heat transfer coefficient were investigated with the operating conditions: vapor quality from 1 to 0, mass flux from 160 to 450 kg/(m2s), heat flux from 10.0 to 39.8 kW/m2, and saturation pressure from 600 to 1500 kPa. The experimental results indicated that the pin fin array significantly improves the condensation heat transfer coefficient up to nearly 186 % compared to the multiport mini channel in the high vapor quality region. For both channels, the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas decreases with increase in saturation pressure. The influence of heat flux and mass flux on heat transfer coefficient was more pronounced in the high vapor quality region than in the low vapor quality region. A sensitivity analysis was performed at different vapor quality levels. The most influential parameters in the smooth channel and the pin fin array are saturation pressure and mass flux, respectively. The relative contribution of heat flux is more obvious in the high vapor quality region. Some of the existing correlations have good prediction performance for the smooth channel experimental data in this paper, and the corresponding MAD is within 20 %. However, their deviations are much greater for the applications of pin fin array.

Development and validation of a general two-fluid thermal-hydraulic analysis code for reactors with plate-type fuel assemblies

A general two-fluid thermal-hydraulic analysis code, TTHAC-PFA (Two-fluid Thermal-Hydraulic Analysis Code for reactors using Plate-type Fuel Assemblies), has been developed. The code was developed on basis of the fundamental governing equations. Reasonable mathematic and physical models for parallel rectangular channels and fuel plates have been built up to meet the command of the safety analysis for plate-type fuel reactors. The coolant distribution is conducted on basis of the pressure drop equation theory. Normal subchannel approach is adopted in the code. Fortran language is adopted. The RA-6 experiment is used to evaluate the heat transfer models. The max deviation of the simulations from the measured value is about 5.7 °C. Then, experiments on two-phase flow pressure drop are adopted to validate the code. Approximately 97% of the data points are within ±20% of the deviation range. In addition, CARR (China Advanced Research Reactor) is adopted as the benchmark for normal condition validation. Flow distribution validation and temperature profile validation are conducted. The flow validation presents negligible mean discrepancy and the temperature differences are within the scope of 6 °C. All validations are in good agreement, which indicates the developed code meet the requirements of analyzing thermal-hydraulic characteristics for parallel rectangular channels in reactors. Furthermore, TTHAC-PFA is applied to the investigate channel blockage accident occurred in the CARR. The mass flow is redistributed among channels due to the obstruction. In the obstructed channel and two nearby channels, the coolant temperatures increase a lot. The coolant temperature in the obstructed channel rises by 60 °C. In the rest of the channels, the coolant temperatures decrease with the increase of coolant flow. Besides, the two fuel plates, which form the blocked channel, are asymmetrically cooled due to flow maldistribution. The temperature difference between the cladding on both sides of the fuel plate can reach 40 °C. As a result of partial blockage, the cladding temperature on the side of the blocked channel rises by 63 °C. During the blockage accident, the plate temperature is under the melting temperature. The results indicate the significance of studying the effects of the adjacent channels near the obstructed channel for reactors with plate-type fuel elements.

Flow regime and Reynolds number variation effects on the mixing behavior of parallel flows

The hydraulic single-phase mixing of three parallel rectangular channels is experimentally investigated at various Reynolds numbers (Re) and flow regime combinations. Particle Image Velocimetry results for seven mixing cases are presented and discussed with varying Re combinations ranging from 1,824 to 20,844. While all cases result in the same Re ratio of ∼0.69 between the inner and outer flows, two cases represent multi-regime mixing with the inner-outer regime pair of laminar-transitional and transitional-turbulent, while the other 5 cases are all characteristic of turbulent mixing with varying levels of turbulence. The outer channels initially share characteristics with a backward facing step. The center channel is found to initially behave like a slot jet, but then sees a significant increase in velocity decay. This inner flow velocity decay increased dramatically in the laminar-transitional mixing case, whose centerline velocity decay was ∼6 times larger than the decay in the turbulent mixing cases. Second order statistics revealed a consistent mixing layer thickness of ∼0.1 hydraulic diameters for all the cases but showed more intense shearing in the multi-regime mixing cases. The combined point and thereby the mixing layer length is determined using centerline velocity decay profiles, which show a much more aggressive mixing in multi-regime flows. Multi-regime mixing demonstrated superior characteristics relative to turbulent mixing due to a more dramatic velocity decay in the inner flow and a shorter mixing length. The contributions of this work include communicating the benefits of multi-regime mixing and providing detailed characterization efforts that can serve future efforts for validating computational models. This research also lays the groundwork for future studies aimed at achieving high levels of mixing without a severe penalty in pressure drop.

Heat transfer characteristics of flow boiling in a micro channel array with various inlet geometries

Flow boiling in micro channels is considered to be a promising solution for electronics cooling. However, its development is impeded by the lack of full understanding of mechanisms responsible for heat exchange and its susceptibility to instabilities. A common way of flow stabilization is employing restrictions at channel inlet, but their impact on micro channel thermal performance remains an open question. Therefore, this paper focuses on experimental determination of the dominant heat transfer mechanism during flow boiling of R245fa in 10 parallel rectangular channels with hydraulic diameter of 1 mm without inlet restrictions and with 0.5 mm wide and 2.5 mm long restrictions. Studied heat fluxes vary between 30 and 50 kW/m2, mass fluxes between 400 and 1000 kg/m2s, saturation temperatures between 49 and 82 ∘C, inlet subcoolings between 13 and 19.4 K. For the subcooled flow the dominant heat transfer mechanism was forced convection with heat transfer coefficient rising with mass flux, vapor quality and saturation temperature. For the saturated flow nucleate boiling was the dominant mechanism with heat transfer coefficient increasing with heat flux and saturation temperature and decreasing with mass flux and vapor quality. Geometry with inlet restrictions in general exhibited similar thermal behavior, but in the subcooled zone a more pronounced positive dependence on saturation temperature and a negative dependence on mass flux especially at lower temperatures were observed. This indicates that restrictions augment nucleate boiling contribution to the forced-convection-dominated heat exchange in the subcooled zone. Restrictions yielded similar or slightly lower heat transfer coefficients. Comparing the data against 24 heat transfer models showed that the model of Thiangtham et al. was the most accurate with MAPE 23.7% and 20.2% for geometry without and with restrictions respectively.

Theoretical and experimental investigation for developing a gas-liquid two-phase flow meter

Despite the intricacy, inline metering of two-phase flow has a significant impact in multitudinous applications including fusion reactors, oil, nuclear, and other cryogenic systems. Since measurement of individual flow rate is prominent in various systems, it warrants the establishment of a flow meter system that can monitor the mass flow rates of liquid. In this regard, an approach was taken towards the development of a two-phase flow meter system in the present study. The concept involves two-phase flow through narrow parallel rectangular channels resulting in laminar, stratified flow with a slope at the liquid-vapor interface. The height of the liquid column at specific channel locations is measured for determining the flow rate. However, the geometric configurations of the channels and fluid properties are pivotal in ensuring accurate measurement. Consequently, theoretical and experimental studies are performed to investigate the correspondence between flow rate and change in liquid height. Based on the governing equations, a theoretical model is established using MATLAB®. The model investigated the intricate influence of various flow and fluid properties in the estimation of the mass flow rate. The experimental investigation was done with various conditions under different liquid and vapor volume flow rates for validating the proposed supposition and the theoretical model. Both the theoretical and experimental analyses showed fair correspondence. The proposed system estimated the mass flow rate within a tolerance of ±10% and showed potential towards the development of the cryogenic two-phase flow meter.

Theoretical research on two-phase flow instability in parallel channels under periodic heaving motion condition

A theoretical model for Density Wave Oscillations (DWOs) flow instability in parallel rectangular channels under periodic heaving motion condition is established with a lumped mathematical model based on homogenous hypothesis. The parallel channel comprises of parallel channels, which consists of entrance section, heating section and riser section, and the upper- and lower plenums guaranteeing the isobaric pressure drop condition between channels. The lumped model consists of boiling channel model, pressure drop model, parallel channel model, additional pressure drop model introduced by periodic heaving motion and the constitutive model and the final nonlinear ordinary differential equations are solved by Gear method. The model is validated with experimental data of a single channel with constant pressure drop and those of twin-tube-channel and twin-rectangular-channel flow instability experiment under static condition, respectively. Afterwards the flow instability in twin-rectangular channel system is studied under periodic heaving motion condition. The effects of amplitude and frequency of heaving motion are examined through the parameter plane of the subcooling and phase change number and the threshold power for flow instability. The results show that the effect of amplitude on flow instability can be divided into three regions in which thermal hydraulic parameters, heaving motion or both of them together dominant the flow instability; the frequency of heaving motion strongly effects the threshold power when it is identical to that of the DWOs, which can be explained by resonance between the heaving motion and DWOs. And this effect is even stronger when the asymmetric throttling and heating condition is introduced.

Investigation of thermal enhancement factor in micro pulsating heat exchanger using LIF visualization technique

This study investigates the thermal flow in an asymmetric micro pulsating heat exchanger (MPHE). In the design and fabrication of the MPHE, the MEMS processes were used to fabricate symmetric and asymmetric MPHEs on which 14 and 16 parallel rectangular channels were formed, respectively. Ethanol mixing with fluorescence dye (rhodamine B) was used as a working fluid in order to visualize the thermal flow by a laser induced fluorescence technique. The fluid columns inside the micro channel were changed rapidly by deforming the interface of liquid slug and gas in the asymmetric MPHE. Thermal resistance was evaluated under various operating conditions. The asymmetric MPHE filled with 60% working fluid has the highest heat transfer efficiency under vertical orientation. From the thermal flow visualization inside the asymmetric MPHE filled with ethanol, the annular flow by dispersing and generating was identified as the key mechanism of circulating motion. The circulating motion of asymmetric channels supports the heat transfer more actively than the oscillating motion of symmetric channels.

Analytical, experimental, and numerical analysis of a microchannel cooling system for high-concentration photovoltaic cells

In this work, we present the results of an analytical, numerical, and experimental analysis on the performance of a heat sink system designed as a parallel arrangement of microchannels for cooling a high-concentration photovoltaic (HCPV) cell. The analysis considered the worst-case scenario where no electricity is generated, and the solar incidence is maximum on the northwest region of the Sao Paulo State in Brazil. For the experimental, analytical, and numerical analysis, the considered HCPV cell has a geometrical concentration ratio of 500×, a maximum efficiency of 40% at cell’s operating temperature of 41.0 °C, and a cell base area of 100 mm2. The numerical analysis adopts the finite volume method implemented in ANSYS Fluent v15 to solve flow and energy equations with second-order upwind schemes, and the steady-state, incompressible, and laminar flow. In the experimental apparatus, the copper microchannel heat sink consists of 33 parallel rectangular channels of 10 mm in length, 200 μm in width, and 500 μm in height for each microchannel. A cartridge heater was used to simulate the on-sun test, i.e., it simulates the total heat rate supplied to the microchannel heat sink. The microchannel heat sink is capable of keeping the operating temperature of the cell below the maximum cell’s operating temperature (41.0 °C). In addition, the pressure drops are slightly higher than the predicted models, but not exceeding 34%. Moreover, the energy spent in the pumping in the microchannel represents < 1% of the energy generated by the photovoltaic cell.

A comprehensive criterion for slug-annular flow transition based on flow boiling of R134a in microchannels

A microchannel test loop with an air cooling system was established in this study. Experiments were conducted with R134a within 30 parallel rectangular channels of 0.5 mm × 0.5 mm × 45 mm. The heat flux was in the range of 49.9 to 580.1 kW/m2 and the mass flux was in the range of 132 to 1529 kg/m2 s in this research, respectively. The saturation temperature in the range of 21.6 to 32.9 °C. The heat transfer coefficient increased with the mass flux and showed a curve like “M”-shape with the increase of heat flux. Under the same conditions of heat and mass flux, the higher saturation temperature was, the lower wall superheat was, and the heat transfer coefficient was improved. The G-x flow patterns map showed that a higher saturation temperature resulted in a lower vapor quality in the channels, which dedicated to the flow patterns distribution in the nuclear boiling region; whereas a lower saturation temperature led to a higher vapor quality, and the convective evaporation dominated. Based on the influences of multiple factors on the slug-annular flow transition in microchannels, a new predicting correlation for the vapor quality at the valley point of the “M”-shape was proposed, which provided a criterion for the transition from nuclear boiling mechanism into convective evaporation mechanism.

Flow behavior of rapid thermal oscillation inside an asymmetric micro pulsating heat exchanger

This study has investigated the rapid thermal oscillatory flow in an asymmetric micro pulsating heat exchanger (MPHE). In the design and fabrication of the MPHE, a transparent acrylic plate was attached to the stainless steel plate on which 14 parallel rectangular channels were formed. As working fluids, FC-72 and ethanol were tested. The fluid inside the micro channel moves rapidly with pulsating motions by swelling and shrinking of liquid slug/gas plug at evaporation and condensation sections. By measuring the temperature variations from start-up to steady state operation, the effective thermal conductivities were evaluated. Since the filling ratio of working fluid plays an important role in heat exchanger efficiency, the optimum filling ratio was found for each working fluid. From the flow visualization inside the MPHE filled with FC-72, two different flow modes – oscillatory eruption mode and circulation mode – occurs according to the filling ratio. The circulating motion which occurs at the optimum filling ratio, helps the heat transfer into the cooling water more active than the oscillatory eruption mode.

Experimental investigation on the flow boiling of R134a in a multi-microchannel heat sink

The present study reports on an experimental evaluation of flow boiling of R134a inside a multi-microchannel heat sink. The copper test section consisted of 27 parallel rectangular channels with 0.470mm depth, 0.382mm width, 0.416mm fin thickness and 40mm length. The experiments were performed at saturation temperatures of 18, 23 and 28°C, mass fluxes of 800, 1000 and 1200kgm−2s−1 and constant vapor quality at the heat sink inlet at 0.05. The wall heat flux was increased from 50kWm−2 until reaching critical heat flux, and maximum value of 460kWm−2 was reached. With the collected data from the experiments, the effects of mass flux, heat flux, saturation temperature and steam quality on the heat transfer coefficient are emphasized and the reasons are discussed. At low vapor quality, heat flux plays a major role in increasing the heat transfer coefficient, while the effect of mass flux is negligible. With rising heat flux, convective boiling begins to control the heat transfer mechanism, and the heat transfer coefficient increases with rising vapor quality and mass flux, until dry-out point. In all conditions, high heat transfer coefficients are obtained for high saturation temperatures. In open literature, the correlation generated by Mortada et al. for heat transfer coefficient is the one that makes the most accurate prediction for current study. Based on the experimental results, a correlation is proposed to calculate the heat transfer coefficients for R134a flow boiling inside the multi-microchannel at high mass and heat flux predicting the measured ones better than those in the literature.

Visualization of R134a flow boiling in micro-channels to establish a novel bubbly-slug flow transition criterion

In this study, the fundamental flow pattern characteristics of R134a flow boiling in a micro-channel heat sink were investigated based on high-speed flow visualization. Visual experiments were operated with R134a as the working fluid. The test section was composed with 20 parallel rectangular channels 500 μm wide, 500 μm deep, and 60 mm long. Mass flux and heat flux were variable from 164 to 573 kg/m2 s and 1 kW/m2 to 200 kW/m2, respectively, at the inlet temperature of 22 °C. Flow images were recorded with speed up to 10,000 frames/s. Bubbly flow, slug flow, churn flow, liquid lump flow, and annular flow were observed experimentally and the results plotted onto an x-G flow pattern map. For “M”-shaped heat transfer coefficient curve, five observed flow patterns were combined into one curve to reflect the heat transfer characteristics of flow boiling in micro-channels. Mechanisms of transition regions (bubbly-slug, slug-churn, churn-liquid lump, and liquid lump-annular) were established accordingly. The Active Nucleation Site Density Na was found to be the most significant parameter in regards to the bubbly-slug and slug-churn transition mechanisms. To verify the effect of the Active Nucleation Site Density (Na) on the bubbly-slug transition, the relations of Na and Boiling number at bubbly and slug flow regions were compared; the resulting slope features made the relations of Na and Boiling number easily distinguishable. Classification between the bubbly flow and slug flow of R134a was proposed based on these distinctions. The proposed criterion classified the transition region between the bubbly and slug flow. The proposed transition criterion was proven effective and feasible by comparison against previous transition criteria and datasets.

The effect of heating direction on flow boiling heat transfer of R134a in micro-channels

This paper presents effects of heating directions on heat transfer performance of R134a flow boiling in micro- channel heat sink. The heat sink has 30 parallel rectangular channels with cross-sectional dimensions of 500μm width 500μm depth and 30mm length. The experimental operation condition ranges of the heat flux and the mass flux were 13.48 to 82.25 W/cm2 and 373.3 to 1244.4 kg/m2s respectively. The vapor quality ranged from 0.07 to 0.93. The heat transfer coefficients of top heating and bottom heating both were up to 25 kW/m2 K. Two dominate transfer mechanisms of nucleate boiling and convection boiling were observed according to boiling curves. The experimental results indicated that the heat transfer coefficient of bottom heating was 13.9% higher than top heating in low heat flux, while in high heat flux, the heat transfer coefficient of bottom heating was 9.9%.higher than the top heating, because bubbles were harder to divorce the heating wall. And a modified correlation was provided to predict heat transfer of top heating.

An experimental study on two-phase flow patterns and heat transfer characteristics during boiling of R134a flowing through a multi-microchannel heat sink

The present paper presents an experimental study of flow patterns and heat transfer characteristics of flow boiling of R134a in a multi-microchannel heat sink. The copper test section has 27 parallel rectangular channels with a depth of 470μm, a width of 382μm, a length of 40mm, and a fin thickness of 416μm. The experimental results are presented for saturation temperatures of 13, 18, and 23°C, and mass fluxes of 150, 400, and 600kg/m2s. The wall heat flux and inlet vapor quality values were between 3 and 127kW/m2, and 0.05 and 0.92, respectively. The effects of pertinent parameters on the heat and fluid flow characteristics such as saturation temperature, mass flux, heat flux, and inlet vapor quality are studied and discussed. The heat transfer coefficient at high saturation temperatures (i.e. 23°C) is higher than low saturation temperatures (i.e. 13°C) in the heat flux range of 40–120kW/m2. For high heat flux ranges, the heat transfer coefficient increases with increasing mass flux. The convective boiling heat transfer mechanism will play a major role in wavy and annular flow patterns. For wall heat fluxes higher than 80kW/m2, the existence of a partial dry-out phenomenon in the multi-microchannel leads to a decrease in heat transfer coefficient. The results unveil the significant effect of flow patterns on heat transfer characteristics. Based on the experimental data, a correlation is proposed to calculate the heat transfer coefficient for R134a flow in the multi-microchannel heat sink that is useful in electronic cooling applications.

Design, fabrication and test of an integrated multi-microchannel heat sink for electronics cooling

In this paper, MEMS fabrication techniques are used to assemble a micro device which integrates a simulated heat source, a multi-microchannel heat sink and flow sensors.The front side of the silicon chip comprises one 1000nm-thick Aluminum heater and six 400Å-thick Ruthenium thermal sensors microfabricated to achieve precise and interface-free heat flux imposition and temperature measurements, respectively. The heat sink on the backside is made of 27 parallel rectangular channels with hydraulic diameter Dh=96μm, length to diameter ratio L/Dh=104 and relative roughness of 2.4%. The channels are closed with glass to allow visualization of the flow, and sealed by anodic bonding, instead of using adhesives, thermal fusion or mechanical sealing, in order to guarantee resistant sealing over the entire range of operating pressures and temperatures while still preserving the depth of the microchannels.Primary fabrication tests are performed to evaluate the influence of geometrical parameters (thin film width and length) in thermal response and joule heating performance for surface temperatures in the range from 25°C to 106°C obtained with currents from 0mA to 250mA. The temperature coefficient of electrical resistivity, TCR, of the Ru thermal sensors is found to be (1.72±0.45)×10−3K−1. The thermo-fluid-dynamic performance of the device is assessed with hydrofluoroether coolant (HFE-7000) with Reynolds numbers up to 640 (mass flux up to 2727kgm−2s−1) and constant wall heat fluxes q″ up to 10.8Wcm−2. Preliminary tests have shown that, without boiling, the temperature of the chip can be kept below 85°C (Tw,avg=65.4°C) with a maximum heat flux of 10.8Wcm−2 and a flow rate of 3050mLh−1. Boiling of the coolant fluid allows to increase the heat flux up to 26.6Wcm−2 with a flow rate of 1300mLh−1 (Re=277) with perfect robustness and reliability of the system.

Heat transfer enhancement in a PV cell using Boehmite nanofluid

Experiments were conducted to investigate the cooling performance of channels by water-based nanofluids containing small concentrations of Boehmite (AlOOH⋅xH2O) for the PV cell. The channels were fabricated in two different configurations as straight and helical. The straight channel consists of 23 parallel rectangular channels with a hydraulic diameter of 4.1mm, a length of 24.5cm, a width of 5mm and a depth of 3.5mm. For the helical configuration the plate was divided into four symmetrical sections which, each part contains a helical channel with a width of 5mm, a depth of 3.5mm and a hydraulic diameter of 4.1mm. The nanofluid cooling performance was assessed from the results obtained for the average temperature and electrical output of the PV cell. Results showed that the nanofluid perform better than water and caused higher decrease in the average PV cell temperature. They were about 39.70% and 53.76% for 0.1wt.% (the best case) at flow rate of 80ml/min for the helical and straight channel, respectively. Moreover, the highest decrease of average temperature relative to the reference temperature was observed for 0.1wt.% nanofluid concentration in both channels. This led to the highest electrical efficiency about 20.57% and 37.67% for the straight and helical channel, respectively. Furthermore, power-hydraulics performance was defined to compare the performance of the studied channels. The results showed that the helical channel works in more efficient way compared with the straight one.

Flow boiling heat transfer of R407C in a microchannels based heat spreader

New flow boiling experimental results for R407C in a microchannel based heat spreader are presented. Boiling curves were obtained for heat fluxes up to 310kW/m2 (based on the footprint area), mass velocities from 400 to 1500kg/m2s, liquid subcoolings at the test section inlet of 5, 10 and 15°C and saturation temperatures referred to the pressure at the heat sink inlet of approximately 25°C. Based on these results, heat sink averaged heat transfer coefficients during convective boiling were estimated. The heat sink evaluated in the present study is comprised of fifty parallel rectangular channels with cross-section dimensions of 100×500μm, and total length of 15mm. Average heat transfer coefficients up to 30kW/m2°C were obtained. It was also found that the boiling curve moves to the left hand side with decreasing the mass velocity and liquid subcooling at the heat-sink inlet. Moreover, for a fixed heat-sink averaged vapor quality, the average heat transfer coefficient increases with increasing mass velocity. Under similar experimental conditions, the refrigerant R134a provided higher heat transfer coefficients than R407C. Additionally, during flow boiling of R407C, pressure oscillations with lower amplitude and frequency were observed compared to R134a. No one of the heat transfer predictive methods evaluated in the present study was accurate enough to predict the present R407C database.

Comparison of flow instabilities under static condition and marine motion conditions based on experiments

An experimental investigation was performed on two-phase flow instability with twin parallel rectangular channels under static condition and marine motion conditions. Marine motion conditions include inclination, fluctuation and rolling. The maximum inclination angle is 45°, maximum amplitude and acceleration of fluctuation are 0.8m and 1.9m2/s, respectively; maximum amplitude and angular acceleration of rolling are 30° and 0.5rad/s2, respectively. Test thermal parameters are 3–8MPa for pressure, 300–700kg/m2s for mass velocity, and 15–90°C for inlet subcooling in this experiment. Comparisons have been made on four aspects: (1) general observation of flow oscillation; (2) period of flow oscillation; (3) flow instability boundary heat flux; (4) flow instability zone under static condition and motion conditions. Contrary to what is observed under static condition, the flow oscillation under motion conditions is actually the superposition of thermal-induced oscillation and motion-induced oscillation. But further comparison indicates that the influence of marine motion conditions on flow stability is very limited. In general, the period of oscillation, flow instability boundary heat flux and distribution of instability zone are much more affected by thermal parameters than the motion conditions and the point to point comparison shows that the differences of boundary heat flux brought by motion is no more than ±5%. Finally, an empirical formula considering a pressure correction is correlated based on experimental data, which is applicable for the prediction of the instability boundary under both static and motion conditions.