The effects of micro-structured surfaces on multi-nozzle spray cooling

Experiments were conducted to investigate heat transfer characteristics of spray cooling with eight nozzles for micro-structured surfaces included cubic pin fins and straight pin fins of different sizes. Liquid volume flow rate ranged from 2.46 × 10⁻² m³/s/m² to 3.91 × 10⁻² m³/s/m² and the corresponded inlet pressures changed from 0.28 MPa to 0.6 MPa by keeping the inlet water temperature between 20.4 °C and 24.31 °C. And the input power of heat block varied from 180 W to 1080 W. The results show that the heat transfer performances of straight fins2 and straight fins3 are the best in single phase zone, but the cubic pin fins is better in two phase zone. Notably, the critical point between single phase zone and two phase zone shifts to left with the increasing of liquid volume flow rate. Moreover, with the liquid volume flow rate increasing, the heat transfer coefficient increases as well, but straight fins1 and polished surface are not sensitive to this change. For a deeper analysis of the heat transfer enhancement, a dimensionless number (DM) is created to characterize heat transfer performance of different microstructures in single phase heat transfer. We verified the dimensionless number using experimental results in this study and previous literature. Furthermore, the micro-structured surfaces have negligible effects on temperature distribution except for cubic pin fins.     

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.     

Spray cooling of enhanced surfaces: Impact of structured surface geometry and spray axis inclination

Experiments were conducted to study the effects of enhanced surfaces and spray inclination angle (the angle between the surface normal and the axis of symmetry of the spray) on heat transfer during spray cooling. The surface enhancements consisted of cubic pin fins, pyramids, and straight fins. These structures were machined on the top surface of heated copper blocks with 2.0 cm² cross-sectional areas. Measurements were also obtained on a heated flat surface to provide baseline data. PF-5060 was used as the working fluid. The spray was produced using a 2 × 2 nozzle array under nominally degassed conditions (chamber pressure of 41.4 kPa) with a volume flux of 0.016 m³/m² s and a nozzle height of 17 mm. The spray temperature was 20.5 °C. For the geometries tested, the straight fins had the largest heat flux enhancement relative to the flat surface, followed by the cubic pin fins and the pyramid surface. Each of these surfaces also indicated an increase in evaporation efficiency at CHF compared to the flat surface. Inclination of the spray axis between 0° and 45° relative to the heater surface normal created a noticeable increase in heat flux compared to the normal position (0° case). A maximum heat flux enhancement of 23% was attained for the flat surface. The straight finned surface had a maximum heat flux enhancement of 75% at an inclination angle of 30° relative to the flat surface in the normal position. However, only a marginal increase (11%) was observed in comparison to the straight finned surface in the normal position (0° case).     

Modeling of water spray evaporation: Application to passive cooling of buildings

The use of water droplet evaporation in shower towers and passive downdraft evaporative cooling needs the estimation of the time needed to completely evaporate the drops. To solve this problem, a cellular approach is proposed in which the spray is considered as a pile of rigid spheres of equal size; each sphere has multiple layers and contains a drop in its center. The evaporation takes place gradually from the superficial layer towards the internal layers. Parametric studies show the influence of each variable on the evaporation time of the droplets. The model may be used for sizing passive evaporative cooling systems and towers for buildings using the passive evaporative down draught effect. A building equipped with a shower tower has been tested in the framework of the European project PDEC/JOULE in Catania (Italy). The spraying system was successfully sized by using the model proposed in this paper.     

Mist/steam cooling by a row of impinging jets

Mist/steam cooling has been studied to augment internal steam-only cooling for advanced turbine systems. Water droplets generally less than 10 μm are added to 1.3 bar steam and injected through a row of four round jets onto a heated surface. The Reynolds number is varied from 7500 to 22,500 and the heat flux varied from 3.3 to 13.4 kW/m². The mist enhances the heat transfer along the stagnation line and downstream wanes in about 3 jet diameters. The heat transfer coefficient improves by 50-700% at the stagnation line for mist concentrations 0.75-3.5% by weight. Off-axis maximum cooling occurs in most of the mist/steam flow but not in the steam-only flow. CFD simulation indicates that this off-axis cooling peak is caused by droplets’ interaction with the target walls.     

Effects of spray characteristics on critical heat flux in subcooled water spray cooling

Effects of spray parameters (mean droplet size, droplet flux, and droplet velocity) on critical heat flux (CHF) were studied while these parameters were systematically varied. The effect of each parameter was studied while keeping the other two nearly constant. The mean droplet velocity (V) had the most dominant effect on CHF and the heat transfer coefficient at CHF (h_c), followed by the mean droplet flux (N). The Sauter mean diameter (d₃₂) did not appear to have an effect on CHF. By increasing V, CHF and h_c were increased. This trend was observed when all other spray parameters were kept within narrow ranges and even when relaxed to wider ranges, indicating the dominant effect of V. The effect of N, although not so much as V, was also found to be significant. Increasing N resulted in an increase in CHF and h_c when other parameters are kept in narrow ranges. A dilute spray with large droplet velocities appears to be more effective in increasing CHF than a denser spray with lower velocities for a given N. The mass flow rate was not a controlling parameter of CHF.     

The impact of various nanofluid types on triangular microchannels heat sink cooling performance

This paper discusses the impact of using various types of nanofluids on heat transfer and fluid flow characteristics in triangular shaped microchannel heat sink (MCHS). In this study, an aluminum MCHS performance is examined using water as a base fluid with different types of nanofluids such as Al₂O₃, Ag, CuO, diamond, SiO₂, and TiO₂ as the coolants with nanoparticle volume fraction of 2%. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite volume method. It is inferred that diamond-H₂O nanofluid has the lowest temperature and the highest heat transfer coefficient, while Al₂O₃-H₂O nanofluid has the highest temperature and the lowest heat transfer coefficient. SiO₂-H₂O nanofluid has the highest pressure drop and wall shear stress while Ag-H₂O nanofluid has the lowest pressure drop and wall shear stress among other nanofluid types. Based on the presented results, diamond-H₂O and Ag-H₂O nanofluids are recommended to achieve overall heat transfer enhancement and low pressure drop, respectively, compared with pure water.     

An experimental investigation of heat transfer characteristics for steam cooling in a rectangular channel with parallel ribs

An experimental study of heat transfer characteristics in superheated steam cooled rectangular channels with parallel ribs was conducted.The distribution of the heat transfer coefficient on the rib-roughed channel was measured by IR camera.The blockage ratio(e/Dh) of the tested channel is 0.078 and the aspect ratio(W/H) is fixed at3.0.Influences of the rib pitch-to-height ratio(P/e) and the rib angle on heat transfer for steam cooling were investigated.In this paper,the Reynolds number(Re) for steam ranges from 3070 to 14800,the rib pitch-to-height ratios were 8,10 and 12,and rib angles were 90°,75°,60°,and 45°.Based on results above,we have concluded that:In case of channels with 90° tranverse ribs,for larger rib pitch models(the rib pitch-to-height ratio=10 and12),areas with low heat transfer coefficient in front of rib is larger and its minimum is lower,while the position of the region with high heat transfer coefficient nearly remains the same,but its maximun of heat transfer coefficient becomes higher.In case of channels with inclined ribs,heat transfer coefficients on the surface decrease along the direction of each rib and show an apparent nonuniformity,consequently the regions with low Nusselt number values closely following each rib expand along the aforementioned direction and that of relative high Nusselt number values vary inversely.For a square channel with 90° ribs at Re= 14800,wider spacing rib configurations(the rib pitch-to-height ratio=10 and 12) give an area-averaged heat transfer on the rib-roughened surface about8.4%and 11.4%more than P/e=8 model,respectively;for inclined parallel ribs with different rib angles at Re=14800,the area-averaged heat transfer coefficients of 75°,60° and 45° ribbed surfaces increase by 20.1%,42.0%and 44.4%in comparison with 90° rib angle model.45° angle rib-roughened channel leads to a maximal augmentation of the area-averaged heat transfer coefficient in all research objects in this paper.     

Effects of titania nanoparticles on heat transfer performance of spray cooling with full cone nozzle

Spray cooling using aqueous titania nanofluids was studied. The temperatures of a testing plate under various spraying conditions were first measured; an inverse heat conduction technique was then applied to convert these measured temperatures into heat transfer coefficients (HTCs). It was found that the HTC increased logarithmically with the volume flux, but was decreased with the increase of the nanoparticle fraction. A correlation analysis was performed to quantify the HTC reduction caused by the increase of nanoparticles, and reconfirmed that the major cause for the HTC reduction was the difference in the impact (or impingement) behavior between solid nanoparticles and fluid droplets. A comparison study of the present findings with the previous published results was also performed and indicated that all results compared were consistent to each other based on the similar spray cooling conditions with different nanofluids or nozzles. The effects by using aquatic titania nanofluids instead of aquatic alumina nanofluids and by using full-cone nozzle instead of solid jet nozzle were specifically assessed and the associated rationales for the differences in these effects were given.     

Heat transfer correlation for intermittent spray impingement: A dynamic approach

The work presented here investigates a new approach in the development of heat transfer empirical correlations for intermittent spray impingement, based on simultaneous measurements of the spray droplets characteristics and the surface thermal behavior. Conventionally, heat transfer correlations for spray impingement do not consider the temporal variations of droplets characteristics. However, in applications using intermittent sprays (internal combustion engines, cryogen spray cooling or microprocessor thermal management), the spray transient behavior suggests that heat transfer predictions may be improved using a dynamic approach. Additionally, the impact of multiple consecutive injections on a heated surface implies a certain degree of interaction, depending on the frequency of their intermittency. If the time between consecutive injections is shorten, the result is the formation of a liquid film which mitigates phase-change and privileges a single-phase heat transfer over a two-phase. This suggests that heat transfer correlations for spray impingement should take the spray unsteadiness and the multiple injections interaction degree into account. The dynamic approach here suggested presupposes the identification of systematic periods characterizing the spray dynamic behavior and, once identified, the development of a heat transfer correlation for each period. The analysis ends with a comparison between the dynamic heat transfer correlation with a correlation obtained using the conventional approach and a significant improvement in heat transfer predictions is achieved if the spray dynamic nature is considered.     

Heat transfer enhancement and pressure drop of the horizontal concentric tube with twisted wires brush inserts

In the present study, the heat transfer characteristics and the pressure drop of the horizontal concentric tube with twisted wires brush inserts are investigated. The inner diameters of the inner and outer tubes are 15.78 and 25.40 mm, respectively. The twisted wire brushes are fabricated by winding a 0.2 mm diameter of the copper wires over a 2 mm diameter of two twisted iron core-rod with three different twisted wires densities of 100, 200, 300 wires per centimeter. The plain tube with full-length twisted wires brush and regularly spaced twisted wires brush with 30 cm spacer length inserts are tested. Cold and hot water are used as working fluids in shell side and tube-side, respectively. The test runs are performed at the hot water Reynolds number ranging between 6000 and 20000. The inlet cold and hot water temperatures are 15, 20 °C, and between 40 and 50 °C, respectively. Effect of twisted wires density, inlet fluid temperature, and relevant parameters on heat transfer characteristics and pressure drop are considered. Twisted wire brushes insert have a large effect on the enhancement of heat transfer, however, the pressure drops also increase.     

Blowing models for cooling surfaces

To study the cooling of surfaces exposed to high temperature stress and heat flux, the blowing, or transpiration, technique is numerically investigated in the case of a porous circular cylinder. Two models are developed to simulate the blowing impact on the outer flow and an experimental set-up available allows for direct comparison and validation of the numerical simulations. The heat exchange occuring within the porous wall itself between the coolant and the solid part of the wall is accounted. The results show an excellent effectiveness of the blowing in terms of surface temperature reduction, even for low blowing ratii. The incident heat flux exhibits a maximum for medium blowing rates due to a decreasing heat transfer coefficient and a growing temperature difference between the surface and the main flow with the injection rate. Finally, the blowing is demonstrated to be very effective in cooling heavily thermally stressed parts in terms of homogeneity and coolant rate required.     

Effects of chemical reaction caused by cooling stream on film cooling effectiveness

With the increase of inlet temperature of gas turbines, the benefits by using the conventional methods are likely to approach their limits. Therefore, it is essential to study novel film cooling methods for surpassing these current limits. Based on the theory of heat transfer enhancement, a film cooling method with chemical reaction by cool- ing stream is proposed. In order to test the feasibility of the proposed method, numerical simulations have been conducted. The classic flat plate structure with a 30 degree hole is used for the simulation. In the present study, the effects of the parameters in relation to the chemical reaction on film cooling effectiveness, such as chemical heat sink, volume changes, and reaction rate, are investigated numerically. The conventional film cooling is also calculated for the comparison. The results show that film cooling effectiveness is improved obviously due to the chemical reaction, and the reaction heat and reaction rate of cooling stream have an important effect on film ef- fectiveness. However, the effect of volume changes can be ignored.     

Exergy transfer effectiveness on heat exchanger for finite pressure drop

In this paper, exergy transfer effectiveness is defined to describe the performance of heat exchangers operating above/below the surrounding temperature with/without finite pressure drop. It is discussed systemically that the effects of heat transfer units number, the ratio of the heat capacity of cold fluids to that of hot fluids and flow patterns on exergy transfer effectiveness of heat exchangers. Furthermore, the results of exergy transfer effectiveness with a finite pressure drop are compared with those without pressure drop when different objective media, such as ideal gas and incompressible liquid, etc. are applied. The detailed comparisons of the exergy transfer effectiveness with heat transfer effectiveness are also performed for the parallel flow, counter flow and cross flow heat exchangers operating above/below the surrounding temperature.     

An exact heat transfer analysis of spherical products subjected to forced-air cooling

The thermal analysis of forced-air cooling processes being of primary concern, an experimental and analytical study program was undertaken to investigate the heat transfer during the cooling of figs as spherical food products. The process conditions were analysed according to a mathematical model to gain a better understanding of the product's behaviour. The heat transfer between the product and air was influenced by conduction inside the product, convection outside the product, radiation, respiratory heat rate (internal heat generation), and moisture evaporation at the surface of the product. These situations were considered as three cases, such as h = h_c, h = h_c + h_c, and h = h_c, + h_r + h_e. The four various air velocities of 1.1, 1.5, 1.75, and 2.5 m/s were applied in the experimental study. The results obtained by the mathematical model in the estimation of the heat transfer rates from the products were compared with the experimental data, and the best agreement was found for the third case (h = h_c + h_r + h_e). The fastest cooling was accomplished with the highest airflow velocity.     

Thermal-fluid assessment of multijet atomization for spray cooling applications

Thermal management is a particularly difficult challenge to the miniaturization of electronic components because it requires high performance cooling systems capable of removing large heat loads at fast rates in order to keep the operating temperature low and controlled. To meet this challenge, the Intermittent Spray Cooling (ISC) concept has been suggested as a promising technology which uses a proper match between the frequency and duration of consecutive injection cycles to control heat transfer. This concept also depends on: the atomization strategy; a homogeneous dispersion of droplets impinging on the hot surface; and the quantitative control of the liquid deposited, avoiding excessive secondary atomization or pre-impingement-evaporation. In this work, the use of liquid atomization by multiple jets impact, also referred as multijet atomization, is the subject of a thermal-fluid assessment using heat transfer correlations previously derived for intermittent sprays. Simultaneous measurements of droplet size and velocity are provided as input for the correlations and the analysis explores the influence of the number of impinging jets on the heat removal pattern and magnitude. Emphasis is put on the promising applicability of multijet atomization for promoting an intelligent use of energy in the thermal management of electronic devices.     

闭式冷却塔内冷却盘管传热热阻分析

通过对闭式冷却塔内冷却盘管各热阻的数量级分析,认为在实际计算中,管壁导热热阻比其它热阻小一个数量级,计算中可忽略,但其余热阻均不可忽略。影响盘管总热阻大小的因素很多,从数学上分析了各热阻对总热阻的影响,找出影响盘管总热阻的主次因素,为冷却盘管的研究、设计及运行管理提供参考。     

Computational fluid dynamic investigation of liquid rack cooling in data centres

Relying on thermal air management in a data centre is becoming less effective as heat densities from the Information Technology (IT) equipment continue to rise. Direct liquid cooling is more efficient at transferring the waste heat, but requires liquid loops passing as close as possible to the heat source. A new Computational Fluid Dynamics (CFD) strategy is developed for data centre scenarios where a liquid loop heat exchanger is attached at the rear of server racks (back doors), which can avoid the need to separate the cold and hot air streams in traditional hot/cold aisle arrangements. The effectiveness of additional fans in the back door heat exchangers is investigated using the three-dimensional CFD model of a simplified three-aisle, six-rack data centre configuration.     

Experimental study of turbulent convective heat transfer and pressure drop of dilute CuO/water nanofluid inside a circular tube

Turbulent convective heat transfer performance and pressure drop of very dilute (less than 0.24% volume) CuO/water nanofluid flowing through a circular tube were investigated experimentally. Measurements showed that addition of small amounts of nanosized CuO particles to the base fluid increased heat transfer coefficients considerably. In average 25% increase in heat transfer coefficient was observed with 20% penalty in pressure drop. Enhancement ratio did not show significant variation with concentration of CuO in nanofluid in the range studied in this work.