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The influence of nucleation on the flow boiling heat transfer coefficient of R-134a/R-290/R-600a refrigerant mixture is experimentally
studied in a smooth horizontal tube of 12.7 mm diameter. The heat transfer coefficients are experimentally measured for stratified
flow patterns under a varied heat flux condition; a condition found in the evaporator of refrigerators and deep freezers.
The experiments are conducted in a counter-current heat exchanger test section. By regulating the flow rate and inlet temperature
of acetone, which is the heating fluid flowing in the outer tube, a varied heat flux is provided to the refrigerant flowing
in the inner tube. The refrigerant mass flow rate is fixed between 3 and 5 g s−1 and its inlet temperature between −8.59 and 5.33°C, which corresponds to a pressure of 3.2 to 5 bar. The significance of
nucleate boiling prevailing in the above-mentioned evaporators is highlighted. The experimental heat transfer coefficients
are also compared with well known heat transfer correlations. 相似文献
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以R600a压力式封闭系统喷雾冷却过程为研究对象,对其换热过程进行分析。对液滴撞击热面后的状态进行建模,分析了其运动状态。通过忽略液膜的对流换热,引入韦伯数来简化并修正雾滴与热源表面的对流换热系数关联式;借鉴二次成核理论,通过单位时间内,单位面积上覆盖的雾滴数量对核态沸腾换热系数关联式修正。通过上述分析,以对流换热和核态沸腾换热两种机理为中心,建立了新的换热系数关联式。通过与其他文献的关联式、实验测量值进行比较、不同工质进行比较、不同实验系统比较,发现该式预测值和实验测量值偏差在±20%以内,能够很好地预测压力式封闭系统喷雾冷却过程的换热系数。 相似文献
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This study presents new data on nucleate boiling heat transfer obtained in compact horizontal tube bundles with small tube gaps. The experiment investigates the heat transfer enhancement effects by the restricted spaces comprising the compact tube bundles and the enhanced heat transfer tubes for nucleate boiling heat transfer of R-11 at atmospheric pressure. A roll-worked tube was used as a new type of enhanced heat transfer tube. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer in a smooth tube bundle, while enhancement effects of small gaps were not quite significant for the enhanced tube bundle. There is a compound effect from the enhanced surface and the restricted space only for the enhanced tube bundle with the tube gap of 0.5 mm. The effects of the tube positions within both compact tube bundles on the boiling heat transfer were minor. 相似文献
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Spray cooling is an effective tool to dissipate high heat fluxes from hot surfaces. This article thoroughly investigates the effect of thickness of a hot stainless steel plate on the cooling time, cooling rate, heat flux, and heat transfer coefficient under constant mass flow rate maintained at 1 MPa using water as the coolant. Cylindrical samples of stainless steel with constant diameter (D = 25 mm) and thickness (δ = 7.5, 12, 16.5, and 21 mm) were used in the present study. Critical droplet diameter to achieve an ultra-fast cooling rate of 300°C/s was estimated by using an analytical model for samples of varying thicknesses. The analytical model (one side spray cooling) showed good agreement with experimental results with a relative error of 3.2% in the plate thickness range of 1–12 mm. An increasing trend in maximum heat flux was found with increasing thickness of the plate. Maximum heat flux as high as 1,800 kW/m2 was achieved for a 21-mm-thick sample. Heat transfer coefficients in the range 0.092–96.24 kW/m2K, 0.111–98.9 kW/m2K, 0.074–63.4 kW/m2K, and 0.127–55.63 kW/m2K were reported for sample of varying thicknesses in the present study. Limited published work is available with reference to water spray cooling dynamics and thickness of stainless steel plate. Therefore, the present study focuses on the correlation between the thickness of the plate and spray dynamics of water spray cooling. 相似文献
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Abstract The mechanisms of nucleate boiling on the outside of a horizontal tube differ fundamentally from those on a flat plate. In this experimental work the variation of heat transfer coefficient around the periphery of a tube is measured with the aim of clarifying these mechanisms. A specially designed tube is used in which local variations are not masked by conduction through the metal surface. The tube diameter is 27 mm, and the working fluid is R113 under saturated conditions at 1 atm. When there is no imposed velocity the peripheral variations are typically 10–20% with the maximum heat transfer coefficient at the base of the tube. At very low velocity of upward flow ( ? 0.1 m/s) there is a marked change in the variation, with the maximum coefficient occurring at a point about 70° from the base. At higher velocities there is a slight increase in angle to the maximum point with maximum peripheral variations in heat transfer coefficient of around 25%. The variations are explained in terms of nucleate boiling at the base and top and flow boiling at the sides. The thin layer of two-phase bubbly flow at the sides leads to the predominance of heat transfer caused by sliding bubbles aver other mechanisms in this region. The complex mix of mechanisms involved in boiling on tubes implies an inherent limit to the accuracy of predictive correlations. 相似文献
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ABSTRACTThe present paper reports the experimental investigation of pool boiling heat transfer on multiscale functionalized copper surfaces. Multiscale functionalized surfaces are fabricated by employing the nano-second laser surface process (NLSP) technique. The heat transfer coefficients (HTCs) of functionalized surfaces are estimated experimentally by using water and acetone as pool liquid. Tests are performed at atmospheric pressure, and saturated pool boiling condition with heat flux varyies between 0 and 330 kW/m2. The maximum HTCs for functionalized surface and reference polished surface were found to be 41,500 W/m2K and 23,000 W/m2K, respectively, with water and 22,000 W/m2K and 14,000 W/m2K, respectively, with acetone. 相似文献
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《Heat Recovery Systems and CHP》1992,12(1):39-48
This paper presents an experimental study of flow boiling heat transfer characteristics of refrigerant mixture R-22/R-114 in the annuli of a horizontal enhanced surface tubing evaporator. The geometric parameters of the test section are: inner tube bore diameter of 17.5 mm, envelope diameter of 28.6 mm and outer smooth tube of 32.3 mm I.D. The ranges of heat flux and mass velocity covered in the tests were 5–25 kW m−2 and 180–290 kg m2−s−1 respectively, at a pressure of 570 kPa. The enhanced surface tubing data showed a significant enhancement of the heat transfer compared to an equivalent smooth tube depending on the mixture's components and their concentrations. Correlations were proposed to predict the heat transfer characteristics such as average heat transfer coefficients as well as pressure drops of R-22/R-114 non-azeotropic refrigerant mixture flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition. 相似文献
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对于沸腾换热,一个主要的约束条件就是临界热流密度(Critical Heat Flux,简称CHF)。这个约束条件对沸腾换热量有一个最高值的限制。文中对矩形微槽道中的流动沸腾临界热流密度进行了实验研究。实验数据是在不同尺寸(0.15mm;0.4mm;1mm)微槽道中,在较大范围的面积质量流速和不同进口过冷度下,以去离子水为工质得到的。实验过程中发现,达到CHF时,靠近出口壁面温度会突然升高,此时传热效率迅速下降。实验数据分析结果表明:CHF随质量流量的增加而增加;进口过冷度对CHF没有明显影响;CHF随着出口干度的增加而降低。 相似文献
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A. R. Pati 《实验传热》2019,32(2):116-132
The significant reduction of Leidenfrost effect during the cooling of high carbon steel plate by different potential cooling methodologies does not assure their successful implementation in the fast quenching of high carbon steel plate due to the formation of oxide layer of comparatively low thermal conductivity on the quenching surface. Therefore, the role of oxide layer in case of different potential cooling methodologies needs to be addressed. In the present study, the effect of oxide layer on heat transfer rate in case of upward, downward, and both upward and downward facing spray with additives has been investigated by conducting and comparing the heat transfer cooling data of an AISI 1020 plate with the AISI 304 plate. The comparison clearly depicts that the formation of oxide layer during cooling significantly hinders the heat transfer rate in nucleate boiling regime; however, the reverse phenomenon is observed in transition boiling regime. Among all the coolants, the least effect of oxide layer on enhancement is obtained in case of NaCl (0.4 M)-added water spray due to the deposition of salt on the evaporating surface. The X-ray diffraction analysis and the thickness of the formed oxide layer clearly assert that the coolant depicting minimum oxidation characteristic is preferred.
Abbreviations: AISI: American iron and steel institute; OES: Optical emission spectrophotometer; CHF: Critical heat flux, MW/m2; IHF: Initial heat flux, MW/m2; TCHF: Temperature at which CHF is achieved, °C; Fps: Frames per second; XRD: X-Ray diffraction; k1: Thermal conductivity of steel plate, W/m °C; k2: Thermal conductivity of oxide layer, W/m °C; k3: Thermal conductivity of coolant, W/m °C; X: x-axis, mm; Y: y-axis, mm; Z: z-axis, mm 相似文献
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An experimental investigation has been carried out to study the enhancement in heat transfer coefficient by inserting coiled wire around the outer surface of the inner tube of the double-pipe heat exchanger. Insulated wires, with a circular cross-section of 2 mm diameter, forming a coil of different pitches (p = 6, 12, and 20 mm), were used as turbulators. The investigation is performed for turbulent water flow in a double-pipe heat exchanger with cold water in the annulus space for both parallel and counter flows. The experiments were performed for Reynolds numbers ranging from 4,000 to 14,000. The experimental results reveal that the use of coiled circular wires leads to a considerable increase in heat transfer coefficients compared with a smooth wall tube for both parallel and counter water flows. The mean Nusselt number increases with Reynolds number and pitch. The convective heat transfer coefficient for a turbulent water flow increases for all coiled wire pitches, with the highest enhancement of about 450% for counter flow and 400% for the parallel flow. New correlations for mean relative Nusselt numbers at different coiled wire pitches are provided. 相似文献
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对一种斜翅型外翅片带内螺纹的冷凝强化换热管进行传热性能的实验研究。管外冷凝换热的制冷剂为R134a,管内对流换热的介质为水。分别在定热流密度与定水流速的条件下进行一系列工况的实验,得到相应的实验数据。在定热流密度条件下,利用Wilson图解法得到管内的换热系数数据及相应的计算关联式。在定水流速的条件下,利用分离方法得到管外冷凝换热系数数据及相应的计算关联式。将强化管换热系数数据与光管换热系数的理论计算值进行了比较,结果表明:冷凝强化换热管管内对流换热的强化倍率为2.4,管外凝结换热系数随壁面过冷度的增加而增大,管外凝结换热的强化倍率为:1.78~3.92。 相似文献
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《Heat Recovery Systems and CHP》1994,14(5):507-515
In the frame of the JOULE 1 R&D programme of the Commission of the European Communities a project has been carried out on enhanced evaporation heat transfer surfaces. A specific investigation has been realised on refrigerant oil mixtures boiling in a planar confined space.An experimental investigation of the boiling phenomenon in the confined space between a 30 mm wide × 120 mm high, heated plate and an opposing, adiabatic plate was carried out. The heated surface was made of a copper-aluminium-nickel alloy (Ra = 1 micrometer) and a saturated R-113/SUNISO 3GS oil mixture at atmospheric pressure was used as the boiling fluid.The maximum heat flux tested was approximately 90% of CHF. The parameters investigated were the gap size (1–5 mm) and the oil concentration (1–7% by weight). It was again observed that confinement does not improve the nucleate boiling performance of pure R-113 in any significant way, whereas the CHF decreases with decreasing gap size. In addition, while the presence of oil was observed to have only a relatively minor effect on low flux nucleate boiling, it caused a serious degradation of the high flux boiling performance. This deterioration increased with increasing oil concentration and was more severe for smaller gap sizes. However, for a given gap size, the CHF increased with increasing oil concentration accompanied by increasingly larger surface superheats. 相似文献
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In this article, an experimental investigation is performed to measure the boiling heat transfer coefficient of water flow in a microchannel with a hydraulic diameter of 500 μm. Experimental tests are conducted with heat fluxes ranging from 100 to 400 kW/m2, vapor quality from 0 to 0.2, and mass fluxes of 200, 400, and 600 kg/m2s. Also, this study has modified the liquid Froude number to present a flow pattern transition toward an annular flow. Experimental results show that the flow boiling heat transfer coefficient is not dependent on mass flux and vapor quality but on heat flux to a certain degree. The measured heat transfer coefficient is compared with a few available correlations proposed for macroscales, and it is found that previous correlations have overestimated the flow boiling heat transfer coefficient for the test conditions considered in this work. This article proposes a new correlation model regarding the boiling heat transfer coefficient in mini- and microchannels using boiling number, Reynolds number, and modified Froude number. 相似文献
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Air-atomized spray cooling of a hot moving AISI 304 steel plate of 6 mm thickness has been investigated experimentally by varying water flow rate and plate velocity at a fixed nozzle-to-plate distance. It is found that the heat transfer coefficient is a non-linear function of surface temperature. The result shows that the cooling rate increases with an increase in the water flow rate. The highest cooling rate has been found for the static plate, whereas for a moving plate, an increasing cooling rate trend has been observed with increasing plate velocity. 相似文献