Fast transient spray cooling of a hot thick target (original) (raw)
Related papers
Heat transfer in the film boiling regime: Single drop impact and spray cooling
International Journal of Heat and Mass Transfer, 2017
In this study a model for the heat transfer into a single drop impacting onto a hot solid substrate in the film boiling regime is developed. The model accounts for the expansion of the thermal boundary layers in the spreading drop and in the solid substrate, and for the evaporation of the liquid phase, leading to the creation of a thin vapor layer. An explicit expression for thickness of the vapor layer is obtained from the energy balance at the liquid-vapor and vapor-solid interfaces. The theory allows prediction of the heat transferred from the hot substrate to the single drop during impact. This quantity is then used for the development of a model for an average heat transfer coefficient for spray cooling in the film boiling regime. The model accounts for the probability of drop interactions on the wall, when the droplet number density in the spray is high. The theoretical predictions for the heat transfer coefficient agree well with existing experimental data.
Thermo-and fluid dynamics characterization of spray cooling with pulsed sprays
Experimental thermal and fluid science, 2005
A fundamental study is reported here on the spray impingement of pulsed sprays on heated surfaces. The experiments consider simultaneous measurements of surface heat flux and droplet characteristics performed with a Phase Doppler Anemometer (velocity, size and flux) prior to impact, to provide a better insight into the interaction between thermal and fluid dynamic effects during the period of injection. The experimental conditions are relevant for engines at steady rotational speeds between 1800 rpm and 3600 rpm, which are of interest for hybrid configurations, where the IC engine is set to operate at maximum efficiency. The analysis addresses the effects of injection conditions (e.g., duration, frequency and pressure) on the thermodynamic behaviour of the surface. It is observed that the heat flux decreases when the pressure of injection increases, due to dynamic variations of the film induced by interaction with impacting droplets in the film evaporation regime. Results further suggest that, for the range of injection conditions found in real engines, the time variation of the heat transfer during injection depends more on the liquid mass flux than on droplet size and axial velocity. However, when the engine load increases, the mechanism by which heat is removed from the surface varies from thermally controlled to mass diffusion controlled, due to saturation of the atmosphere with gasoline vapour. The time resolved measurements are processed to quantify the critical points of the boiling curves, e.g., the critical heat flux (CHF) at the Nukiyama temperature and the minimum heat flux (MHF) at the Leidenfrost temperature. The dynamic characteristics of those curves are used as an approach to describe the heat transfer mechanisms in pulsed spray systems. It is suggested that multiple droplet interaction alters the thermal behaviour of the target in the sense that both, CHF and MHF, increase in regions of large droplet concentration and when the frequency of injection increases. However, load variations associated with times of injection in the range 5-10 ms, do not produce significant alterations. Analysis shows the likely importance of transient mechanisms occurring at the solid-liquid and liquid-vapour interface in describing the interaction of pulsed sprays with hot surfaces.
… on Applications of Laser …, 2004
The present paper describes an experimental methodology devised to study spray cooling with pulsed sprays combining a two-component Phase Doppler Anemometer with fast response surface thermocouples. The hardware allows simultaneous acquisition of Doppler and thermocouple signals which are processed in Matlab to estimate the timevarying heat flux and fluid-dynamic characteristics of the spray during impact. The time resolution of the acquisition system is limited by the data rate of validation of the Phase Doppler Anemometer, but it has been shown to be accurate for the characterization of spray-cooling processes with spurt durations of the order of 5 ms.
An experimental methodology to quantify the spray cooling event at intermittent spray impact
… journal of heat and fluid flow, 2007
The present paper describes an experimental methodology devised to study spray cooling with multiple-intermittent sprays as those found in fuel injection systems of spark-ignition and diesel engines, or in dermatologic surgery applications. The spray characteristics and the surface thermal behaviour are measured by combining a two-component phase-Doppler anemometer with fast response surface thermocouples. The hardware allows simultaneous acquisition of Doppler and thermocouple signals which are processed in Matlab to estimate the time-varying heat flux and fluid-dynamic characteristics of the spray during impact. The time resolution of the acquisition system is limited by the data rate of validation of the phase-Doppler anemometer, but it has been shown to be accurate for the characterization of spray-cooling processes with short spurt durations for which the transient period of spray injection plays an important role. The measurements are processed in terms of the instantaneous heat fluxes, from which phase-average values of the boiling curves are obtained. Two of the characteristic parameters used in the thermal analysis of stationary spray cooling events, the critical heat flux (CHF) and Leidenfrost phenomenon, are then inferred in terms of operating conditions of the multiple-intermittent injections, such as the frequency, duration and pressure of injection.
Heat transfer at multiple-intermittent impacts of a hollow cone spray
International journal of heat and mass transfer, 2006
The present work is aimed at quantifying the effects of frequency in the heat transfer at multiple-intermittent impacts of a hollow cone spray in a way to contribute to the development of advanced heat transfer techniques. The flow configuration is that of a spray impacting perpendicularly onto an aluminium flat plate located at 55 mm. The experiments are conducted at prescribed temperatures ranging from local nucleate/boiling to local transition regimes of heat transfer and with frequencies of injection from 10 Hz to 30 Hz with durations of 5 ms. Analysis is based on spatial resolved measurements of the instantaneous surface temperature during the period of injection, processed in order to obtain estimates of the instantaneous heat flux.
Film dynamics relevant to spray cooling
2010
For a number of years spray cooling has shown to be a viable alternative for thermal management of high-density electronics. Nevertheless, the key fundamental physical processes are to a large degree poorly understood due mostly to the complicated fluid dynamics resulting from nucleate boiling coupled with spray drop impingement. In this combined experimental and modeling effort, a representative configuration consisting of a liquid film resting on a solid silicon-based substrate with an imposed constant heat flux and an impinging train of droplets has been studied. This configuration mimics to a great degree the physics of spray cooling, while simultaneously simplifying the experimental and computational analysis to a manageable level. It is shown that a number of statistically quasi-stationary states are possible by carefully coordinating the heat flux and drop impingement rates. Studies were both performed for water and FC-72. Due in part to its lower surface tension, the quasi-stationary states for FC-72 were instantaneously much more chaotic than the corresponding water cases. The OpenFoam (open source computational fluid dynamics) code has been supplemented with an energy equation within the existing Volume-of-Fluid infrastructure. This was used to analyze the dynamics in the impingement region. It is shown that the temperature in this region is approximately equal to the temperature of incident droplets. For all water and FC-72 films, it was found that each droplet impact penetrated the entire thickness of the film bringing a significant cooling effect on the heated substrate. This was the case even for film thickness-to-impact droplet diameter ratios far exceeding one.
From drop impact physics to spray cooling models: a critical review
Experiments in Fluids, 2018
Spray-wall interaction is an important process encountered in a large number of existing and emerging technologies and is the underlying phenomenon associated with spray cooling. Spray cooling is a very efficient technology, surpassing all other conventional cooling methods, especially those not involving phase change and not exploiting the latent heat of vaporization. However, the effectiveness of spray cooling is dependent on a large number of parameters, including spray characteristics like drop size, velocity and number density, the surface morphology, but also on the temperature range and thermal properties of the materials involved. Indeed, the temperature of the substrate can have significant influence on the hydrodynamics of drop and spray impact, an aspect which is seldom considered in model formulation. This process is extremely complex, thus most design rules to date are highly empirical in nature. On the other hand, significant theoretical progress has been made in recent years about the interaction of single drops with heated walls and improvements to the fundamentals of spray cooling can now be anticipated. The present review has the objective of summarizing some of these recent advances and to establish a framework for future development of more reliable and universal physics-based correlations to describe quantities involved in spray cooling.
Interaction of Drops and Sprays with a Heated Wall
Springer eBooks, 2022
Spray-wall interactions take place in many technical applications such as spray cooling, combustion processes, cleaning, wetting of surfaces, coating and painting, etc. The outcome of drop impact onto hot surfaces depends on a variety of parameters like for example material and thermal properties of the liquid and wall, substrate wetting properties, surrounding conditions which determine the saturation temperature, spray impact parameters and surface temperature. The aim of the current project is to improve knowledge of the underlying physics of spray-wall interactions. As an important step towards spray impact modeling first a single drop impact onto hot substrates is considered in detail. Various regimes of single drop impact, such as thermal atomization, magic carpet breakup, nucleate boiling and thermosuperrepellency, observed at different wall temperatures, ambient pressures and impact velocities, have been investigated experimentally and modelled theoretically during the project period. The heat flux, an important parameter for spray cooling, has been modeled not only for single drop impacts but also for sprays within many regimes. The models show a good agreement with experimental data as well as data from literature.
Materiali in Tehnologije, 2015
The cooling of vertically moving strips is used very often to obtain the required material properties. Water spray cooling has to be used when a high cooling intensity is needed. Our Heat Transfer and Fluid Flow Laboratory is equipped with a testing device which allows vertical movement of a heated experimental plate (sheet). Two different sizes of flat-jet nozzles were tested with different water pressures and angles of the water impact (inclination angles of the spraying bar). The water-pressure range was between 2 bar and 9.3 bar and the angle of the water impact changed from 20°to 40°. The dependence of the heat-transfer coefficient on the surface temperature was evaluated for each experiment. Interesting results were obtained from the comparison of these experimental results, showing that the heat-transfer coefficient and the Leidenfrost temperature increase with the increasing water pressure. Very interesting results were obtained during the tests with different inclination angles. The highest heat-transfer coefficient was obtained for the angle of 20°and the lowest value of the heat-transfer coefficient was obtained for the angle of 40°at the surface temperatures of around 200°C.
Gravity effect on spray impact and spray cooling
Microgravity Science and Technology, 2007
An experimental study of the film produced by the spray impact on a heated target and of the spray cooling has been performed in normal gravity and in microgravity conditions during parabolic flights. A convex shape of the target allowed visualization of the film evolution and determination of the film characteristics using the image processing. The effects of the spray parameters and of the gravity level on heat transfer have been investigated. It has been found that the spray cooling efficiency depends on the water flow rate in a nonmonotonous way. A range of spray parameters at which the effect of gravity level on heat transfer is significant has been determined. It has been found that the spray cooling is less effective in microgravity conditions in comparison with normal gravity and hypergravity.