Effect Of Surface And Liquid Thermal Properties On Droplet Impact And Spray Cooling

Recent years has known significant increase in electronics and miniaturization of electronic chips.This development was accompanied with increasing demand in power which led to arise other problems.Among which the problem of heating which limits further development.Heat spray cooling seems to be a promising technology because of its various advantages.Heat spray cooling is the dispersion of a large number of tiny micro-droplets to impact on the hot surface.Understanding the nature of the interaction of a single impacting droplet and the heated surface can help to improve our understanding on the spray cooling which is considered to be a complicated technique.The solid surface on which the droplet impacts is characterized by two major thermal properties,the thermal diffusivity and the thermal effusivity.In the first part of our study we aim to experimentally investigate the effect of the surface thermal diffusivity and effusivity on the Leidenfrost temperature.Our experimental results revealed that the Leidenfrost temperature is affected by the thermal effusivity rather than the thermal diffusivity.We propose a model based on a semi-infinite solid to account for the effect of the thermal effusivity and the impacting velocity on the Leidenfrost temperature.The proposed model shows that the heat flux increases for surfaces with high effusivity and thus,the Leidenfrost state at low surface temperatures can be achieved.This tendency is explained by the intense vapor generation with high pressure for high effusive surfaces leading to insulate the impacting droplet from making contact with the hot surface.In contrast,low effusive surfaces results in low heat flux and generate less vapor with low pressure,the droplet is able to contact the surface.Surfaces with low thermal effusivity requires higher surface temperature for the surface to reach the Leidenfrost state.We also reveal that the difference in the dynamic Leidenfrost temperature between low and high impact velocities is high for surfaces with low thermal effusivity and it decreases with surfaces having high thermal effusivity.This is caused by their very fast vapor generation with high pressure induced by the high thermal effusivity.The results of our developed model and the experimental results show good agreement.In this next section,we try to understand the effect of different liquid properties on droplet Impact.Nanofluids with different thermal properties and concentrations were prepared.Graphene and silica nanofluids were prepared at 0.1 vol% and 0.01 vol% concentration.Graphene nanofluid at0.1 vol% concentration could not be used because of the strong instability and the needle clogging during experiments.Silica nanofluid at 0.01 vol% concentration did not show significant difference as compared with pure alcohol.However,graphene nanofluid with 0.01 vol%concentration shows lower Leidenfrost temperature,as low as 30 °C beside pure alcohol;silica at0.1 vol% concentration shows lower Leidenfrost temperature by 15 °C beside alcohol.Both transitions to the Leidenfrost state happened at small temperature increase by increasing the impact velocity.The decrease in the Leidenfrost temperature is caused by the increase of the thermal conductivity of the nanofluids.Higher conductivity increases the evaporation rate leading to create enough pressure to levitate the droplet at low surface superheats.Low latent heat of evaporation also has important influence for low Leidenfrost temperatures.The maximum droplet spreading was observed to decrease by increasing the surface temperature.Droplet surface contact increases the maximum spreading because of the high fluid wettability.The droplet in the film boiling shows smaller maximum spreading because of the influence of the liquid properties,namely,the surface tension and viscosity.The impacting droplet spreading was also observed to increase at increasing Weber number.The increase could be divided into two sections for two Weber number power fits.At low Weber number,the power fit is 1/3 and the spreading in this section is interplayed between the droplet inertia and the capillarity.At Weber number higher than 125,the power fit is 1/4 and the spreading in this section is interplayed between the inertia of the droplet on one side and the liquid surface tension and its viscosity on the other side,the capillarity has very weak effect at high impact velocity.Next,isothermal droplet impact experiments were performed on surfaces covered with metal foam of different pore densities.Surfaces covered with metal foam can be seen in this first step as surfaces having low thermal effusivity because of their reduced density.Experiments were also performed using surfactant on the plain surface and the metal foams as well.Results show that surfactant droplet has low contact angle and better wettability than water droplet.Droplet maximum spreading showed various results between the plain surface and surfaces covered with metal foams.Metal foam with 800 ppi showed the maximum spreading followed by the plain surface and then metal foam with 300 ppi.The tendency is same for water and 600 PPM SDS.However,in rest,water droplets keep showing a cap on the top surface of the metal foam while600 PPM SDS totally imbibes inside the metal foam.Based on these results,it is supposed that surfaces covered with metal foams could enhance the heat transfer for heat spray cooling experiments.However,our results showed opposite trend.The heat transfer with metal foam is lower than the plain surface.This result is due to the relatively low flow rate used in our experiments,about 90 ml/min.Low flow rate lacks for high inertia to evacuate the excess of liquid that is accumulated within the metal foam.In contrast to droplet impact studies which showed that the gap between the metal foam and the heater surface enhanced the heat transfer;in our heat spray cooling experiments,the gap between the heater surface(the plain surface)and the metal foam played detrimental role for heat transfer.In the heat spray cooling,the very small distance between the heater surface and the metal foam where the vapor evacuation takes place has very low thermal properties,in addition to the accumulation of the water in the metal foam,hence,poor thermal performance.In order to broaden our understanding on heat spray cooling and improve its feasibility for electronic cooling,experiments were performed to understand the thermal performance of the heat spray cooling with a moving nozzle for large areas.The test-rig included an open loop system with a stepping motor for the nozzle movement along the rectangular heater surfaces.Both the flat and structured surfaces were employed.The experimental results revealed that both the nozzle to surface distance and the nozzle moving speed have an influence on the temperature distribution.The temperature variance is smaller at the lower flow rate than at the higher flow rate.For the structured surface,the temperature variance is lower for the shorter fins.At the low heat flux,a higher heat transfer coefficient is observed on the flat surface than on the enhanced surfaces.This is because of the thin water film on the flat surface as compared to the thick water film on the enhanced surfaces due to the capillary action.However,at the high heat flux,the enhanced surfaces show a higher heat transfer coefficient than the flat surface due to the higher evaporation rate from the water film induced by the increased area of the fins.