Experimental Result on Heat Transfer during Quenching of Hot Steel Plate by Spray Impingement

The present article discusses the experimental results on cooling characteristics of a stationary hot steel plate by spray impingement. The experimental setup consisted of an electrically heated flat stationary steel plate of dimension 120 mm × 120 mm × 4 mm, spray setup, water supply, and air supply unit. The effects of various controlling parameters such as air-water pressures, water flow rate, nozzle tip to target distance and impingement density were determined and analyzed. The cooling rates were computed from the time-dependent temperature history and used to analyze the parametric effects. The results obtained in the study confirmed the higher efficiency of the spray cooling system and the cooling strategy was found advantageous over the conventional cooling methods available in the open literature.     

Effect of Impingement Density and Nozzle to Target Distance on Spray Cooling of Steel Plate—An Experimental Investigation

In the steel making industry, high heat fluxes are obtained using effective cooling techniques such as spray impingement cooling. Spray impingement technique involve factors like droplet size, spray height and spray angle, impingement density, and nozzle geometry rendering it very difficult to measure the effects of individual parameters. In the present study, the cooling rate of the plate was experimentally investigated using distilled water as coolant in 3 pressurized nozzles for spray over the surface of the steel plate at elevated temperatures and the behavior of plate temperature with time was tabulated. Cooling curves were generated for different and varying spray parameters like water pressures, nozzle tip to surface distance, and impingement density. It was observed that the cooling rate at the stagnation zone was strongly dependent on the water pressures and nozzle tip to surface distances with maximum cooling rates reaching within 1–2 seconds after the impingement. The average impingement density increased with increase in water pressure and the cooling rate reduces at higher pressures and nozzle tip to surface distances.     

Monodisperse Spray Cooling of Small Surface Areas at High Heat Flux

Spray cooling is an effective method to remove high heat fluxes from electronic components. To understand the physical mechanisms, this work studies heat transfer rates from single and dual nozzle distilled water sprays on a small heated surface (1.3 mm × 2 mm). Thermal ink jet atomizers generate small droplets, 33 μm diameter, at known frequencies, leading to controlled spray conditions with a monodisperse stream of droplets interacting with the hot surface. Of particular interest in this work is the dissipated heat flux and its relation to the liquid film thickness, the surface superheat, and the cooling mass flow rate. Experimental results show the heat flux scales to the cooling mass flow rate. In comparison to published spreading–splashing correlations, these experiments indicate that the drops impinge on the liquid film and spread without generating splashing, leading to high-efficiency stable heat transfer. Surface temperatures range from 120 to 140°C. In addition, the liquid film thickness is investigated in relation to the heater superheat and a stable thin film is seen at superheats beyond 20°C. The efficiency of the spray system is inversely related to the film thickness and may be due to ejection of liquid from the surface due to bursting of vapor bubbles.     

Experimental study of the effects of structured surface geometry on water spray cooling performance in non-boiling regime

Experiments were conducted to study the effects of enhanced surfaces on heat transfer performance during water spray cooling in non-boiling regime. The surface enhancement is straight fin. The structures were machined on the top surface of heated copper blocks with a cross-sectional area of 10 mm×10 mm. The spray was performed using Unijet full cone nozzles with a volumetric flux of 0.044–0.053 m³/(m²·s) and a nozzle height of 17 mm. It is found that the heat transfer is obviously enhanced for straight fin surfaces relative to the flat surface. However, the increment decreases as the fin height increases. For flat surface and enhanced surfaces with a fin height of 0.1 mm and 0.2 mm, as the coolant flux increases, the heat flux increases as well. However, for finned surface with a height of 0.4 mm, the heat flux is not sensitive to the coolant volumetric flux. Changed film thickness and the form of water/surface interaction due to an enhanced surface structure (different fin height) are the main reasons for changing of the local heat transfer coefficient.     

Cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film

The cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film were experimentally investigated in order to estimate the cooling performance of a rotating roll in a hot mill system. The following four conclusions were reached in the study. (1) In the case of a single spray jet, the local heat transfer coefficient at the center position depends on the forced convective heat transfer by the impinging jet. However, the average heat transfer coefficient is proportional to the flow rate density of the cooling water, and it does not depend on the distance between the nozzle and heated surface. (2) In the case of a double spray jet, liquid film interference occurs. The local heat transfer coefficient at the center position is greater, and the cooling performance increases with the increasing flow rate density of the cooling water. (3) The cooling performance of a multispray jet is proportional to the flow rate density of the cooling water. It does not depend on the nozzle construction, distance, or specifications. Also, there is no relation to the liquid film interference. (4) When the optimum specifications of the spray nozzle are used, thermal analysis of a rotating roll shows that the temperature at a depth of 1.3 mm from the surface is below 130 °C. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 280–299, 2000     

Heat transfer and pressure loss characteristics of very compact heat sinks with plate fins cooled by jets

High-performance and very compact heat sinks have been developed for effective cooling of VLSIs with high heat-generation densities. Their heat transfer and pressure loss characteristics in air-jet cooling have been experimentally studied. The highly compact heat sinks were plate-fin arrays with a very small fin pitch of 0.4–2.0 mm. The rectangular jet nozzle width that gave the highest cooling performance was 30 to 40% of the streamwise length of the heat sinks. The influence of fin height on heat transfer became weak when the ratio of the height to the thickness of the fin exceeded approximately 35. When the air flow rate was constant, the thermal conductance increased as the fin pitch decreased. For a constant fin pitch, heat sinks with smaller fin thickness showed larger thermal conductance at a given blower power consumption. In our experimental range, the heat dissipation rate per unit heat sink volume increased as the base plate area of the heat sink became small. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(6): 399–414, 1998     

Heat transfer characteristics of spray cooling in a closed loop

A closed loop spray cooling test setup is established for the cooling of high heat flux heat sources. Eight miniature nozzles in a multi-nozzle plate are used to generate a spray array targeting at a 1 × 2 cm² cooling surface. FC-87, FC-72, methanol and water are used as the working fluids. Thermal performance data for the multi-nozzle spray cooling in the confined and closed system are obtained at various operating temperatures, nozzle pressure drops (from 0.69 to 3.10 bar) and heat fluxes. It is exhibited that the spray cooler can reach the critical heat fluxes up to 90 W/cm² with fluorocarbon fluids and 490 W/cm² with methanol. For water, the critical heat flux is higher than 500 W/cm². Air purposely introduced in the spray cooling system with FC-72 fluid has a significant influence on heat transfer characteristics of the spray over the cooling surface.     

Investigation of spray cooling: Effect of different heater surfaces under acceleration

Spray cooling characteristics under acceleration conditions for different heat transfer surfaces were experimentally investigated with the aid of an acceleration test bench. Three heater surfaces tested were the flat surface, straight fin surface and porous tunnel surface. Water was used as the coolant and sprayed onto the heater surface at various nozzle heights by nozzles having different flow rates. The results of the fundamental studies show that for all the three surfaces, flow rate as well as nozzle height affects the spray cooling performance under the acceleration condition just like it does under the stationary condition. The influence of acceleration for the three heater surfaces is different and for one specific surface the effect of acceleration is also different due to the change in flow rate or nozzle height. According to the comparison results of the heat transfer performance for three surfaces under acceleration condition, it is observed that the porous tunnel surface has the best performance in all tests, followed by the flat surface in most tests. While for a certain case with a low flow rate and low nozzle height, the spray cooling capability for straight fin surface is better than flat surface.     

Characterisation of the spray cooling heat transfer involved in a high pressure die casting process

A systematic experimental study was conducted to examine the heat transfer characteristics from the hot die surface to the water spray involved in high pressure die casting processes. Temperature and heat flux measurements were made locally in the spray field using a heater made from die material H-13 steel and with a surface diameter of 10 mm. The spray cooling curve was determined in the nucleate boiling, critical heat flux, as well as the transition boiling regimes. The hydrodynamic parameters of the spray such as droplet diameters, droplet velocities, and volumetric spray flux were also measured at the position in the spray field identical to that of the test piece. Droplet size and velocity distribution were measured using a PDA system. A new empirical correlation was developed to relate the spray cooling heat flux to the spray hydrodynamic parameters such as liquid volumetric flux, droplet size, and droplet velocity in all heat transfer regimes. The agreement between experimental data and predicted results is satisfactorily good.     

Spray evaporative cooling to achieve ultra fast cooling in runout table

Spray evaporative cooling, in lieu of conventional laminar jet impingement cooling, has potential to achieve the anomalously high strip cooling rate of Ultra Fast Cooling – 300 °C/s for a 4 mm thick carbon steel strip – in Runout Table of Hot Strip Mill. In the present study, evaporation time of a single droplet impinging on a hot carbon steel strip surface has been analytically evaluated as a function of droplet diameter from fundamental heat transfer perspective based on the premise that a spray can be considered as a multi-droplet array of liquid at low spray flux density. Droplet evaporation time thus evaluated has been used to estimate strip cooling rate achievable in Runout Table of Hot Strip Mill by spray evaporative cooling. The proposed analytical model predicts that it is indeed possible to achieve the ultra-high cooling rate of Ultra Fast Cooling by spray evaporative cooling by suitable reduction of droplet size. A general analytical expression has also been developed to estimate critical droplet size to achieve Ultra Fast Cooling as a function of steel strip thickness. Predictions of the analytical model have been validated using CFD simulation with a modified Discrete Phase Model.     

无沸腾喷雾冷却中流量和喷头高度对换热性能的影响

采用薄膜电阻加热器进行了喷头进口压力，喷头类型，喷头高度对换热系数影响的实验研究。研究了冷却介质的质量流量对换热性能的影响，并测量了同一喷头在不同喷头高度下的换热系数大小。实验发现当喷雾面积近似等于实验用薄膜加热器面积时冷却能力达到最大。根据以上实验结果可以最优化喷雾冷却性能。     

Heat transfer and pressure drop experimental correlations for air–water bubbly flow

In this paper, a novel air–water bubbly flow heat transfer experiment is performed to investigate the characteristics of pressure drop of airflow and heat transfer between water and tubes for its potential application in evaporative cooling. The attempts to reduce the pressure drop while maintaining higher heat transfer coefficient have been achieved by decreasing the bubble layer thickness through the water pump circulation. Pressure drops of air passing through the sieve plate and the bubbling layer are measured for different height of bubble layer, hole–plate area ratio of the sieve plate and the superficial air velocity. Experimental data show that the increase of bubble layer height and air velocity both increase the pressure drop while the effect of the hole–plate area ratio of the sieve plate on the heat transfer coefficient is relatively sophisticated. A pressure drop correlation including the effects of all the tested parameters is proposed, which has a mean absolute deviation of 14.5% to that of the experimental data. Heat transfer coefficients of the water and the outside tube wall are measured and the effects of superficial air velocity, heat flux and bubble layer height are also examined. Through a dimensional analysis, a heat transfer correlation with a mean absolute deviation of 9.7% is obtained based on experimental data.     

重油油枪雾化片损坏理论计算与试验分析

沙特某项目5×660 MW燃油/天然气锅炉,自投产以来多次发生雾化片烧损、断裂问题,为了解决上述问题,对雾化片及螺帽结构、雾化片材质、油枪冷却风风量、燃料风风量等进行分析,改进了油枪螺帽的结构和雾化片材质,雾化片表面平均温度由670.2℃降低至581.1℃,解决了雾化片断裂的问题并基本解决了雾化片烧损问题,提高了锅炉运行的安全性。     

A basic study on humidity recovery by using micro‐porous media (Effects of thermal condition of fluids and geometrical condition of apparatus on transport performance)

Using an experimental apparatus to examine the performance of heat and mass transfer between constant‐temperature water and dry air through a porous plate having extremely small pores, the effects of the thermal conditions in the fluids and the geometric condition of the apparatus on moisture transport were measured. The effects of water temperature, thickness of the porous plate, and channel height of the flowing air on moisture transport are noticeable. However, the effect of air temperature in the channel inlet on moisture transport is slight. In addition, in order to evaluate the degree of air humidity absorption, a parameter called the moisture absorption rate was introduced. The moisture absorption rate was shown to decrease with increasing air velocity and varies only slightly for a plate thickness of 1 mm and decreases for a plate thickness of 3.5 mm with increasing water temperature. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(8): 568–581, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20133     

Analysis of the spray field development on a vertical surface during water spray-quenching using a flat spray nozzle

The aims of this study were (i) to conduct experimental spatially and time-resolved measurements of flow development on large heated surfaces during transient spray cooling operations and (ii) to investigate and discuss the influence of spray cooling mechanisms such as bubble formation and the flow field development of the cooling fluid and how this affects heat transfer. Quartz plates were heated to above 500 °C and then sprayed with pressurised water subcooled to 80 K. High speed images of the quench process were collected at a rate of 3000 Hz making it possible to track the movement of the quench front as the plate cools below the Leidenfrost temperature of the fluid at that location. Observations showed that the relative importance of droplet-surface interactions decreases once the Leidenfrost temperature is reached on the plate: It was found that once the water contacts the surface, a water pool develops rapidly which grows larger as the pool floods the heated surface. Comparisons between the spatial flow development and heat transfer on the plate are made in order to describe these interactions more accurately. This information not only provides crucial input into process simulations, but is also useful to develop theoretical models of fluid–solid interaction describing the wetting of a heated component due to water spraying.     

Heat Transfer Enhancement of Spray Cooling on Flat Aluminum Tube Heat Exchanger

This study provides an experimental analysis on the heat transfer performance of a flat aluminum tube microchannel heat exchanger with spray cooling. The effects of water spraying rate, airflow rate, and relative humidity were investigated. The test results show that the heat transfer performance increased with increasing the water spraying rate but without the penalty of increased flow resistance at low spray conditions. This effect is further enhanced by increasing the water spraying rate. However, when the spraying rate is high, part of the nonevaporated drops attached to the fin surface and formed a liquid film, which caused the flow passage to become narrower. Further increase in the spraying rate resulted in part of the flow passages being blocked by the nonevaporated water drops and caused a region of poor heat transfer. The friction coefficient jumped drastically at this condition. This phenomenon deceased gradually with increasing airflow rate. High inlet air humidity resulted in the water accumulation phenomenon appearing at lower water spraying rates. The evaporative cooling effect decreased and flow friction increased. The test results just described show that the water spray is able to significantly improve the air-side heat transfer performance. The optimum spray rate for each airflow rate must be carefully determined.     

Influence of fan setting height on the cooling performance of a plate‐fin‐type heat sink with microprocessor

The cooling performance of a plate‐fin‐type heat sink equipped with a cooling fan was investigated experimentally. The heat sink was 80 mm long, 43 mm wide, and 24 mm in height (including the 4‐mm‐thick base). The cooling fan was 40 × 40 × 15 mm and was set to direct the air flow vertically in the downstream half of the heat sink. We focused on the influence of the height (which varied from 5 to 20 mm) that the fan was set at, on the heat transfer coefficient of the heat sink. The maximum value of the heat transfer coefficient was achieved at a setting height of 5 mm. At this height, the volumetric heat transfer coefficient was 1.8 times as high as that in a parallel flow under the same fan power. This result indicates that the cooling performance of heat sinks with a cooling fan can be improved by using this kind of compact structure. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 512–520, 2001     

热处理钢板气雾冷却数学模型的开发与应用

针对某钢厂新建的热处理钢板气雾冷却装置,基于有限差分传热仿真计算,开发了相应的热处理钢板气雾冷却数学模型,其中详细考虑了钢板在冷却区内所经历的多种传热边界条件,包括水冲击传热、辊子接触传热、辐射传热和自然对流传热。通过现场的钢板表面温度测试工作表明,模型预测值与实际测试值之间的吻合情况良好,模型具有较高的仿真精度。应用该模型,按照一定的热处理工艺要求(主要指对出口温度和钢板表面/中心温降速率的控制),对气雾冷却区进行了冷却水表的理论设计(即各种厚度规格钢板所对应的冷却水流量),其设计结果已成功地应用于生产现场。     

用于汽轮机排汽冷却的蒸发式冷凝器模型及性能分析

蒸发式冷凝器兼具传热性能好和节水的优势,在大型动力系统冷却中具有广阔的应用前景。建立了蒸发冷凝器的理论分析模型,提出了蒸发式冷凝器用于冷却小型汽轮机排汽的设计方案;获得了喷淋水温度、空气和蒸汽的焓值在冷凝器内沿程的变化规律,并对喷淋水量、配风量和空气温度等影响冷凝器性能的影响因素进行了分析。研究结果对蒸发式冷凝器在火力发电行业的应用具有参考意义。     

空气含量和水侧污垢对凝汽器传热系数影响的数值分析

以N12180凝汽器为例,采用自行编制的程序数值计算了空气浓度和污垢对凝汽器汽侧传热系数、冷却管壁导热系数以及平均传热系数的影响,分析了空气与污垢对传热过程的影响机理。结果表明：凝汽器进口空气浓度从0增加到0．01％时,凝汽器平均传热系数降低30％;0．5mm的污垢厚度将使冷却管壁的导热系数降低98％,凝汽器平均传热系数降低85％。