共查询到20条相似文献,搜索用时 15 毫秒
1.
K. N. Shukla A. Brusly Solomon B. C. Pillai B. Jacob Ruba Singh S. Saravana Kumar 《Heat and Mass Transfer》2012,48(11):1913-1920
Nanofluids are employed as the working medium for a conventional cylindrical heat pipe. A cylindrical copper heat pipe of 19.5?mm outer diameter and 400?mm length was fabricated and tested with two different working fluids. The working fluids used in this study are DI-water and Nano-particles suspension (mixture of copper nano particle and DI-water). The overall heat transfer coefficient of the heat pipe was calculated based on the lumped thermal resistance network and compared with the heat transfer coefficient of base fluid filled heat pipe. There is a quantitative improvement in the heat transfer coefficient using nano-particles suspension as the working medium. A heat transfer correlation was also developed based on multiple regression least square method and the results were compared with that obtained by the experiment. 相似文献
2.
The effect of time-dependent pressure pulsations on heat transfer in a pipe flow with constant temperature boundaries is analysed numerically when the viscosity of the pulsating fluid is an inverse linear function of the temperature. The coupled differential equations are solved using Crank-Nicholson semi-implicit finite difference formulation with some modifications.The results indicate local variations in heat transfer due to pulsations. They are useful in the design of heat exchangers working under pulsating flow conditions. The analytical results are presented for both heating and cooling. The conditions under which pulsating flows can augment the heat transfer are discussed. The results are applicable for heat exchangers with fluids of high Prandtl number. 相似文献
3.
In the present study, the heat transfer characteristics in dry surface conditions of a new type of heat exchanger, namely
a helically coiled finned tube heat exchanger, is experimentally investigated. The test section, which is a helically coiled
fined tube heat exchanger, consists of a shell and a helical coil unit. The helical coil unit consists of four concentric
helically coiled tubes of different diameters. Each tube is constructed by bending straight copper tube into a helical coil.
Aluminium crimped spiral fins with thickness of 0.5 mm and outer diameter of 28.25 mm are placed around the tube. The edge
of fin at the inner diameter is corrugated. Ambient air is used as a working fluid in the shell side while hot water is used
for the tube-side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates
are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. The effects of the inlet conditions of both
working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. The air-side heat transfer
coefficient presented in term of the Colburn J factor is proportional to inlet-water temperature and water mass flow rate.
The heat exchanger effectiveness tends to increase with increasing water mass flow rate and also slightly increases with increasing
inlet water temperature. 相似文献
4.
M. Misale P. Garibaldi J.C. Passos G. Ghisi de Bitencourt 《Experimental Thermal and Fluid Science》2007,31(8):1111-1120
This study reports an experimental investigation related to a rectangular single-phase natural circulation mini-loop, which consists of two horizontal copper tubes (heat transfer sections) and two vertical tubes (legs) made of copper, connected by means of four glass 90° bends. The loop inner diameter is 4 mm. The lower heating section consists of an electrical heating wire made of nicromel on the outside of the copper tube; the upper cooling system consists of a coaxial cylindrical heat exchanger with a water–glycol mixture, set at controlled temperature and flowing through the annulus. The loop has an imposed heat flux in the lower heating section and an imposed temperature in the cooler. The mini-loop was placed onto a table which can assume different inclinations. The parameters investigated during the experiments were: power transferred to the fluid and inclination of the loop. The preliminary results show a stable behaviour with a steady temperature difference across the heat sinks. It has been confirmed that the fluid velocity is very small (order of millimetres per second). 相似文献
5.
以甲醇为工质,采用高速数据采集系统测定了微型热驱动回路在不同运行参数下的压力
及温度脉动,其脉动周期及脉动幅度随蒸发段热流密度的增加而减小. 实验发现,在蒸发段
热流密度较低的情况下,蒸气管中是泡状流或弹状流交替存在,而在蒸发段热流密度较高时,
蒸气管中为环状流. 就位差对热性能的影响进行了详细的实验研究,并在冷凝器空气自
然对流和强迫对流情况下,以加热块温度90${^\circ}$C为上限,得出微通道蒸发器和冷凝
器在不同位差下的最大蒸发段热流密度. 通过对实验现象的观察及分析,以期开发出适用于
未来电子产品高功率需求的微型化电子冷却器. 相似文献
6.
Osot Khonsue 《Heat and Mass Transfer》2012,48(10):1767-1771
In the present experimental investigation, the liquid cooling in the micro channel fin heat sink with and without thermoelectric for central processor unit (CPU) of personal computer. The micro channel heat sinks with two different channel height are fabricated from the aluminum with the length, the width and the base thickness of 28, 40, 2?mm respectively. The de-ionized water is used as coolant. Effects of channel height, coolant flow rate, and run condition of PC on the CPU temperature are considered. The liquid cooling in micro-rectangular fin heat sink with thermoelectric is compared with the other cooling techniques. The thermoelectric has a significant effect on the CPU cooling of PC. The experiments are performed at no load and full load conditions within 60?min after steady state, which the mass flow rate are 0.023, 0.017 and 0.01?kg/s. The results heat transfer rate increase with increasing coolant flow rate and higher channel. When comparing with the other cooling system, cooling system with thermoelectric gives the highest efficiency. However, thermoelectric has the high or low heat transfer rate from heat rejected and cooling capacity conditions. 相似文献
7.
Suh Chyn Pang Haji Hassan Masjuki Md. Abul Kalam Md. Ali Hazrat 《Heat and Mass Transfer》2014,50(1):39-56
Automotive designers should design a robust engine cooling system which works well in both normal and severe driving conditions. When vehicles are keyed-off suddenly after some distance of hill-climbing driving, the coolant temperature tends to increase drastically. This is because heat soak in the engine could not be transferred away in a timely manner, as both the water pump and cooling fan stop working after the vehicle is keyed-off. In this research, we aimed to visualize the coolant temperature trend over time before and after the vehicles were keyed-off. In order to prevent coolant temperature from exceeding its boiling point and jeopardizing engine life, a numerical model was further tested with prolonged fan and/or water pump operation after keying-off. One dimensional thermal-fluid simulation was exploited to model the vehicle’s cooling system. The behaviour of engine heat, air flow, and coolant flow over time were varied to observe the corresponding transient coolant temperatures. The robustness of this model was proven by validation with industry field test data. The numerical results provided sensible insights into the proposed solution. In short, prolonging fan operation for 500 s and prolonging both fan and water pump operation for 300 s could reduce coolant peak temperature efficiently. The physical implementation plan and benefits yielded from implementation of the electrical fan and electrical water pump are discussed. 相似文献
8.
Loop heat pipes are heat transfer devices whose operating principle is based on the evaporation and condensation of a working
fluid, and which use the capillary pumping forces to ensure the fluid circulation. A series of tests have been carried out
with a miniature loop heat pipe (mLHP) with flat evaporator and fin-and-tube type condenser. The loop is made of pure copper
with stainless mesh wick and methanol as the working fluid. Detailed study is conducted on the start-up reliability of the
mLHP at high as well as low heat loads. During the testing of mLHP under step power cycles, the thermal response presented
by the loop to achieve steady state is very short. At low heat loads, temperature oscillations are observed throughout the
loop. The amplitudes and frequencies of these fluctuations are large at evaporator wall and evaporator inlet. It is expected
that the extent and nature of the oscillations occurrence is dependent on the thermal and hydrodynamic conditions inside the
compensation chamber. The thermal resistance of the mLHP lies between 0.29 and 3.2°C/W. The effects of different liquid charging ratios and the tilt angles to the start-up and the temperature oscillation are
studied in detail. 相似文献
9.
M. K. Abdullah N. C. Ismail M. Z. Abdullah M. Abdul Mujeebu K. A. Ahmad Z. Mohd Ripin 《Heat and Mass Transfer》2012,48(6):893-901
Piezoelectric fan is a promising option for cooling microelectronic devices owing to its unique features such as no electromagnetic noise, low power consumption and minimum space requirement. The recent interest is to integrate the piezoelectric fans (piezofans) with heat sink; this idea is widely accepted and researches are still underway. This article presents experimental analysis on the effects of tip gap (δ) and amplitude of piezofan vibration (α) on the heat transfer characteristics of finned heat sinks. Two heat sink configurations, namely A and B (with two and four fins respectively) each of which is arranged with three piezofans, are considered for the study. The transient temperature distributions for cases with and without piezofans are obtained for both the configurations, and compared. The heat transfer coefficient, thermal impedance, Nusselt number and Reynolds number are investigated as functions of δ and α. The effect of α on the fan effectiveness is also analyzed. It is observed that the configuration B has better cooling performance compared to A. Among the tested ranges of δ and α, the case with least tip gap (δ?=?0.03) and highest amplitude (α?=?5.29) is found to be the best; at this setting, the fan effectiveness is increased to almost 4?times compared to the case without piezofans. 相似文献
10.
The heat transfer characteristics of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the heat transfer characteristics of two-phase closed thermosyphon (TPCT) with iron oxide-nanofluids are presented. The TPCT is fabricated from the copper tube with the outer diameter and length of 15, 2000 mm, respectively. The TPCT with the de-ionic water and nanofluids (water and nanoparticles) are tested. The iron oxide nanoparticles with mean diameter of 4-5 nm were obtained by the laser pyrolysis technique and the mixtures of water and nanoparticles are prepared using an ultrasonic homogenizer. Effects of TPCT inclination angle, operating temperature and nanoparticles concentration levels on the heat transfer characteristics of TPCT are considered. The nanoparticles have a significant effect on the enhancement of heat transfer characteristics of TPCT. The heat transfer characteristics of TPCT with the nanofluids are compared with that the based fluid. 相似文献
11.
Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Because
of its importance to many cooling procedures, investigations on basic mechanisms of jet impingement boiling heat transfer
are needed. Until now, most of the experimental studies, carried out under steady-state conditions, used a heat flux controlled
system and were limited by the critical heat flux (CHF). The present study focuses on steady-state experiments along the entire
boiling curve for hot plate temperatures of up to 700°C. A test section has been built up simulating a hot plate. It is divided
into 8 independently heated modules of 10 mm length to enable local heat transfer measurements. By means of temperature controlled
systems for each module local steady-state experiments in the whole range between single phase heat transfer and film boiling
are possible. By solving the two dimensional inverse heat conduction problem, the local heat flux and the corresponding wall
temperature on the surface of each module can be computed. The measurements show important differences between boiling curves
measured at the stagnation line and those obtained in the parallel flow region. At the stagnation line, the transition boiling
regime is characterised by very high heat fluxes, extended to large wall superheats. Inversely, boiling curves in the parallel
flow region are very near to classical ones obtained for forced convection boiling. The analysis of temperature fluctuations
measured at a depth of 0.8 mm from the boiling surface enables some conclusions on the boiling mechanism in the different
boiling regimes. 相似文献
12.
Absorber is an important component in vapor absorption refrigeration system and its performance has greater influence in overall
efficiency of absorption machines. Falling film heat and mass transfer in an absorber is greatly influenced by fluid properties,
geometry of heat exchanger and its operating parameters. This paper presents on the results of experimental studies on the
heat and mass transfer characteristics of a coiled tube falling film absorber, using 1,1,1,2-Tetrafluroethane(R-134a) and
N-N Dimethyl Acetamide (DMAC) as working fluids. The effects of film Reynolds number, inlet solution temperature and cooling
water temperature on absorber heat load, over all heat transfer coefficient and mass of refrigerant absorbed are presented
and discussed. Normalized solution and coolant temperature profiles and refrigerant mass absorbed along the height of absorber
are also observed from the experimental results. The optimum over all heat transfer coefficient for R-134a–DMAC solution found
to be 726 W/m2K for a film Reynolds number of 350. The R-134a vapour absorption rate is maximum in the normalized coil height of 0.6 to
1. 相似文献
13.
Y. M. C. Delaur V. S. S. Chan D. B. Murray 《Experimental Thermal and Fluid Science》2003,27(8):911-926
Whole field velocity and point temperature and surface heat flux measurements were performed to characterise the interaction of a single rising ellipsoidal air bubble with the free convection flow from a heated flat surface immersed in water at different angles of inclination. Two thermocouples and a hot film sensor were used to characterise heat transfer from the surface, while a time-resolved digital particle image velocimetry technique was used to map the bubble induced flow in a plane parallel to the surface. Heat flux fluctuations, preceding and following the bubble passage, were shown to correlate with the variation in both local flow velocities and fluid temperatures. The largest increases in heat transfer were recorded when both flow and temperature effects combined to enhance the convective cooling simultaneously. Such conditions were shown to be most likely met when the block was inclined at 45°, thus forcing the bubble to slide closer to the heated surface and hence to the thermal boundary layer. 相似文献
14.
Shen-Hua Hu Jun-Jie YanJin-Shi Wang Yang LiJi-Ping Liu 《International Journal of Multiphase Flow》2007
This paper experimentally studied the effect of macroscopic temperature gradient on Marangoni condensation of ethanol–water vapor mixtures under a wide range of concentrations. For each concentration, the experiment was performed at different velocities and pressures. An oblique copper block was employed to create surface temperature gradient. The results indicated that local heat flux was varied along transversal condensation surface, which was caused by surface temperature gradient. This difference in heat flux might be attributed to the variation of condensate thickness on condensation surface. In addition, a mean heat transfer coefficient was derived along transversal condensation surface. For low ethanol concentration (0.5%, 1%), the coefficient kept a high value over a relative wide range of vapor-to-surface temperature difference (<10 K) and could be augmented as much as 15% as compared with literature under similar experimental condition. Moreover, the mean heat transfer coefficient generally increased with increasing velocity or pressure for all concentrations of the ethanol–water mixtures. 相似文献
15.
Experimental and numerical studies of natural convection in a single phase, closed thermosyphon were carried out using a
vertical, rectangular enclosure model. Only one vertical plate plays the role of heat transfer surface having 100 mm height
and 100 mm width, and others act as the adiabatic wall made of transparent plexi-glass. The heat transfer surface is separated
into three horizontal zones with an equal height; top 1/3 and bottom 1/3 of the surface are cooling and heating zones, respectively
and intermediate section is an adiabatic zone. Water is used as the working fluid. Variable parameters are distance D between the heat transfer surface and an adiabatic plate opposite to the heat transfer plate, and temperature difference
ΔT between heating and cooling zones. By changing both D and ΔT, three regimes of the natural convection flow; quasi-two-dimensional steady, three-dimensional steady and unsteady flows
are observed by means of thermo-sensitive liquid crystal powder and numerically simulated very well by solving a set of governing
equations.
Received on 17 January 2000 相似文献
16.
Experimental study on axial wall heat conduction for convective heat transfer in stainless steel microtube 总被引:1,自引:0,他引:1
Distilled water and nitrogen gas used as the working fluids flow through the stainless steel microtube with inner diameter
168 μm outer diameter 406 μm. Using the Joule heating, the wall temperature field photos of the microtube is acquired by employing
an IR camera and the temperature and the volume flow rate of the inlet and the outlet of microtube are measured. A correlation
between the axial wall heat conduction and the convective heat transfer is obtained by theoretical analysis based on the experimental
results. The investigative results clearly show that the axial heat conduction can reduce the convective heat transfer in
the stainless steel microtube and the decrement may reach 2% compared to the convective heat transfer when the working fluid
is nitrogen gas, however, the decrement can be neglected for distilled water as the working fluid. 相似文献
17.
Within the electronics industry, high degree of integration and enhanced performance has led to high heat dissipation electronic
devices. This has identified the future development of very high heat flux components. In this paper, a novel and high efficient
diffusion welded heat fin-plate radiator (HFPR) was proposed and designed. Various parameters affect the thermal performance
of HFPR. The effect of three parameters: the working fluid filling ratios (8% < FR < 70%), the vacuum degrees (0.001 Pa < VD < 0.1 Pa),
and the air flow velocities (0.5 m/s < u < 6 m/s) were investigated experimentally. Using distilled water and ethanol as working fluids, a series of tests were carried
out to find the influence of the above parameters on steady-state heat transfer characteristics of HFPR. The experimental
results indicated that the filling ratio and vacuum degree had a significant influence on thermal performance of HFPR. Also
compared with cooling performance using distilled water and ethanol, the HFPR cooling component using distilled water had
a stronger heat dissipation capacity for the same filling ratio. The results also can provide a basis for optimal design of
HFPR structure. 相似文献
18.
N. Bhuwakietkumjohn S. Rittidech A. Pattiya 《Journal of Applied Mechanics and Technical Physics》2012,53(2):224-230
The aim of this study is to investigate the heat-transfer characteristics of a top heat mode closed-loop oscillating heat
pipe with a check valve (THMCLOHP/CV). Water and ethanol are used as the working fluids at various working temperatures. The
results show that the specific heat flux increases significantly when the working temperature increases and when the aspect
ratio of the evaporator length L
e
to the pipe diameter d decreases for the pipe filling ratio varying from 30 to 80%. The maximum specific heat flux equal to 786.34 W/m2 is reached with the use of ethanol as the working fluid at L
e
/d = 25, angle of inclination to the horizontal axis 90°, and filling ratio of 80%. 相似文献
19.
The blade tip region in gas turbine encounters high thermal loads due to temperature difference and hence efforts for high durability and safe operations are essential. Improved and robust methods of cooling are required to downgrade heat transfer rate to turbine blades. The blade tip regions, which are exposed to high gas flow, suffers high local thermal load which are due to external tip leakage. Jet impingement, pin cooling etc. are techniques used for cooling blades. A more usual way is to use serpentine passage with 180-degree turn. In this study, numerical simulation of heat transfer distribution of a two-pass square channel with rib turbulators and bleed holes were done. Periodical rib turbulators and bleed holes were used in the channel. The ribs arrangement were 60 degree V rib, 60 degree inverted V ribs, combination of 60 degree V rib at inlet and 60 inverted V rib at outlet section and combination of Inverted V at inlet and V rib at the outlet. The results were numerically computed using Fluent with Reynolds number of 12,500 and 28,500. Turbulence models used for computations were k-ω-SST and RSM. Temperature based and shear stress based techniques were used for heat transfer distribution prediction. The results for 60 degree V rib, 60 degree inverted V ribs were compared with the experimental results for validation of the results obtained. Detailed distribution shows distinctive peaks in heat transfer around bleed holes and rib turbulator. Comparisons of the overall performance of the models with different orientation of rib turbulator are presented. It is found that due to the combination of 60 degree inverted V rib in inlet and 60 V rib in outlet with bleed holes provides better heat treatment. It is suggested that the use of rib turbulator with bleed holes provides suitable for augmenting blade cooling to achieve an optimal balance between thermal and mechanical design requirements. 相似文献
20.
Comparative evaluation of three heat transfer enhancement strategies in a grooved channel 总被引:1,自引:0,他引:1
Results of a comparative evaluation of three heat transfer enhancement strategies for forced convection cooling of a parallel
plate channel populated with heated blocks, representing electronic components mounted on printed circuit boards, are reported.
Heat transfer in the reference geometry, the asymmetrically heated parallel plate channel, is compared with that for the basic
grooved channel, and the same geometry enhanced by cylinders and vanes placed above the downstream edge of each heated block.
In addition to conventional heat transfer and pressure drop measurements, holographic interferometry combined with high-speed
cinematography was used to visualize the unsteady temperature fields in the self-sustained oscillatory flow. The locations
of increased heat transfer within one channel periodicity depend on the enhancement technique applied, and were identified
by analyzing the unsteady temperature distributions visualized by holographic interferometry. This approach allowed gaining
insight into the mechanisms responsible for heat transfer enhancement. Experiments were conducted at moderate flow velocities
in the laminar, transitional and turbulent flow regimes. Reynolds numbers were varied in the range Re = 200–6500, corresponding
to flow velocities from 0.076 to 2.36 m/s. Flow oscillations were first observed between Re = 1050 and 1320 for the basic
grooved channel, and around Re = 350 and 450 for the grooved channels equipped with cylinders and vanes, respectively. At
Reynolds numbers above the onset of oscillations and in the transitional flow regime, heat transfer rates in the investigated
grooved channels exceeded the performance of the reference geometry, the asymmetrically heated parallel plate channel. Heat
transfer in the grooved channels enhanced with cylinders and vanes showed an increase by a factor of 1.2–1.8 and 1.5–3.5,
respectively, when compared to data obtained for the basic grooved channel; however, the accompanying pressure drop penalties
also increased significantly.
Received on 5 April 2001 相似文献