Effects of High Frequency Droplet Train Impingement on Spreading-Splashing Transition, Film Hydrodynamics and Heat Transfer

2016 ◽  
Vol 138 (2) ◽  
Author(s):  
Taolue Zhang ◽  
Jorge Alvarado ◽  
J. P. Muthusamy ◽  
Anoop Kanjirakat ◽  
Reza Sadr
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Liquid Film ◽  
High Frequency ◽  
High Heat ◽  
High Heat Flux ◽  
Droplet Impingement ◽  
Dry Out ◽  
The Impact

The objective of this study is to investigate the effects of droplet-induced crown propagation regimes (spreading and splashing) on liquid film hydrodynamics and heat transfer. In this work, the effects of high frequency droplet train impingement on spreading-splashing transition, liquid film hydrodynamics and surface heat transfer were investigated experimentally. HFE-7100 droplet train was generated using a piezo-electric droplet generator at a fixed flow rate of 165 mL/h. Optical and IR images were captured at stable droplet impingement conditions to visualize the thermal physical process. The droplet-induced crown propagation transition phenomena from spreading to splashing were observed by increasing the droplet Weber number. The liquid film hydrodynamics induced by droplet train impingement becomes more complex when the surface was heated. Bubbles and micro-scale fingering phenomena were observed outside the impact crater under low heat flux conditions. Dry-out was observed outside the impact craters under high heat flux conditions. IR images of the heater surface show that heat transfer was most effective within the droplet impact crater zone due to high fluid inertia including high radial momentum caused by high-frequency droplet impingement. Time-averaged heat transfer measurements indicate that the heat flux-surface temperature curves are linear at low surface temperature and before the onset of dry-out. However, a sharp increase in surface temperature can be observed when dry-out appears on the heater surface. Results also show that strong splashing (We = 850) is unfavorable for heat transfer at high heat flux conditions due to instabilities of the liquid film, which lead to the onset of dry-out. In summary, the results show that droplet Weber number is a significant factor in the spreading-splashing transition, liquid film hydrodynamics and heat transfer.

scholarly journals R&D on divertor plasma facing components at the Institute for Plasma Research

Nukleonika ◽  
2015 ◽  
Vol 60 (2) ◽  
pp. 285-288 ◽  
Author(s):  
Yashashri Patil ◽  
S. Khirwadkar ◽  
S. M. Belsare ◽  
Rajamannar Swamy ◽  
M. S. Khan ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Infrared Thermography ◽  
High Heat ◽  
High Heat Flux ◽  
Acceptance Criteria ◽  
Plasma Facing Components ◽  
Type Test ◽  
Plasma Research

Abstract This paper is focused on various aspects of the development and testing of water cooled divertor PFCs. Divertor PFCs are mainly designed to absorb the heat and particle fluxes outflowing from the core plasma of fusion devices like ITER. The Divertor and First Wall Technology Development Division at the Institute for Plasma Research (IPR), India, is extensively working on development and testing of divertor plasma facing components (PFCs). Tungsten and graphite macro-brush type test mock-ups were produced using vacuum brazing furnace technique and tungsten monoblock type of test mock-ups were obtained by hot radial pressing (HRP) technique. Heat transfer performance of the developed test mock-ups was tested using high heat flux tests with different heat load conditions as well as the surface temperature monitoring using transient infrared thermography technique. Recently we have established the High Heat Flux Test Facility (HHFTF) at IPR with an electron gun EH300V (M/s Von Ardenne Anlagentechnik GmbH, Germany) having maximum power 200 kW. Two tungsten monoblock type test mock-ups were probed using HHFTF. Both of the test mock-ups successfully sustained 316 thermal cycles during high heat flux (HHF) tests. The test mock-ups were non-destructively tested using infrared thermography before and after the HHF tests. In this note we describe the detailed procedure used for testing macro-brush and monoblock type test mock-ups using in-house transient infrared thermography set-up. An acceptance criteria limit was defined for small scale macro-brush type of mock-ups using DTrefmax value and the surface temperature measured during the HHF tests. It is concluded that the heat transfer behavior of a plasma facing component was checked by the HHF tests followed by transient IR thermography. The acceptance criteria DTrefmax limit for a graphite macro-brush mock-up was found to be ~3°C while for a tungsten macro-brush mock-up it was ~5°C.

Cooling of a Heated Surface by Mist Flow

1994 ◽  
Vol 116 (1) ◽  
pp. 167-172 ◽  
Author(s):  
S. L. Lee ◽  
Z. H. Yang ◽  
Y. Hsyua
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Liquid Film ◽  
Heated Surface ◽  
High Heat ◽  
Dynamic Interaction ◽  
High Heat Flux ◽  
Steady Temperature ◽  
Mist Flow ◽  
High Level

Cooling requirements in modern industrial applications, such as the removal of heat from electronic equipments, often demand the simultaneous attainment of a high heat flux and a low and relatively uniform and steady temperature of the heated surface to be cooled. The conventional single-phase convection cooling obviously cannot be expected to function adequately, since the heat flux there is directly proportional to the temperature difference between the heated surface and the surrounding medium. To maintain a high heat flux, the temperature of the heated surface usually must be kept at a high level. An attractive alternative is cooling by a spray, which takes advantage of the significant latent heat of evaporation of the liquid. However, in conventional industrial spray coolings, such as in the case of the cooling tower of a power plant, the temperature of the heated surface usually remains relatively high and is nonuniform and unsteady containing numerous flashy hot spots. In order to optimize the performance of the spray cooling of a heated surface by a mist flow, a clear understanding is required of (1) the dynamic interaction between the droplets and the carrier fluid and (2) the thermal reception of the droplets at the heated surface. It is the dynamic interaction between the phases that is causing the droplets to deposit onto the heated surface. The thermal reception at the heated wall develops mass and heat transfer leading to the mode of cooling of the heated surface. In the present study, an experimental investigation was made of the combination of the dynamic depositional behavior of droplets in a water droplet-air mist flow with the use of a specially designed particle-sizing two-dimensional laser-Doppler anemometer. Also, the heat transfer characteristics at the heated surface were investigated in relation to droplet deposition on the heated surface for wide ranges of droplet size, droplet concentration, mist flow velocity, and heat flux. It was discovered that over a certain suitable range of combination of these parameters, a superbly effective cooling scheme could be established by the evaporation on the outside surface of an ultrathin liquid film. Such a film was formed on the heated surface by the continuous deposition of fine droplets from the mist flow. Under these conditions, the heat flux is primarily related to the evaporation of the ultrathin liquid film on the heated surface and thus depends less on the temperature difference between the heated surf ace and the ambient mist flow. The heated surface is quenched to a low, relatively uniform and steady temperature at a very high level of heat flux. Heat transfer enhancement as high as seven times has been found so far. This effective heat transfer scheme is here termed mist cooling.

Experimental and Numerical Characterization of Droplet-Induced Spreading-Splashing Transition in Surface Cooling

2016 ◽  
Author(s):  
Taolue Zhang ◽  
J. P. Muthusamy ◽  
Jorge Alvarado ◽  
Anoop Kanjirakat ◽  
Reza Sadr
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Weber Number ◽  
High Speed ◽  
Single Phase ◽  
Surface Heat ◽  
Droplet Impingement ◽  
Surface Heat Transfer ◽  
Dry Out

The effects of droplet train impingement on spreading-splashing transition and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of HFE-7100 droplet train was generated using a piezo-electric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet impingement velocity. A thin layer of Indium Tin Oxide (ITO) was coated on a translucent sapphire substrate, which was used as heating element. High-speed and infrared imaging techniques were employed to characterize the hydrodynamics and heat transfer of droplet train impingement. Numerically, the high frequency droplet train impingement process was simulated using ANSYS-Fluent with the Volume of Fluid (VOF) method [1]. The heat transfer process was simulated by applying constant heat flux conditions on the droplet receiving surface. Droplet-induced spreading-splashing transition behavior was investigated by increasing the droplet Weber number while holding flow rate constant. High speed crown propagation images showed that at low-Weber number (We < 400), droplet impingements resulted in smooth spreading of the droplet-induced crown. However, within the transitional droplet Weber number range (We = 400–500), fingering and splashing (i.e. emergence of secondary droplets) could be observed at the crown’s rim. At high droplet Weber number (We > 800), breakup of the crown was observed during the crown propagation process in which the liquid film behaved chaotically. Droplet-induced spreading-splashing transition phenomena were also investigated numerically. Reasonable agreement was reached between the experimental and numerical results in terms of crown morphology at different droplet Weber number values. The effects of spreading-splashing transition on surface heat transfer were also investigated at fixed flow rate conditions. Time-averaged Infrared (IR) temperature measurements indicate that heat flux-surface temperature curves are linear at low surface temperatures and before the onset of dry-out, which indicate that single phase forced convection is the primary heat transfer mechanism under those conditions. Numerical heat transfer simulations were performed within the single phase forced convection regime only. Instantaneous numerical results reveal that droplet-induced crown propagation effectively convect heat radially outward within the droplet impingement zone. Under high heat flux conditions, a sharp increase in surface temperature was observed experimentally when dry-out appeared on the heater surface. It was also found that strong splashing (We > 800) is unfavorable for heat transfer at high surface temperature due to the onset of instabilities seen in the liquid film, which leads to dry-out conditions. In summary, the results indicate that droplet Weber number is a significant factor in the spreading-splashing transition and surface heat transfer.

Prediction of Refrigerant Flow Boiling Hysteresis With an Augmented Separated-Flow Model

2016 ◽  
Author(s):  
Jianwei Gao ◽  
Hongxia Li ◽  
Saif Almheiri ◽  
TieJun Zhang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Flow Model ◽  
Flow Boiling ◽  
Separated Flow ◽  
High Heat ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Boiling Hysteresis

Thermal management is essential to compact devices particularly for high heat flux removal applications. As a popular thermal technology, refrigeration cooling is able to provide relatively high heat flux removal capability and uniform device surface temperature. In a refrigeration cycle, the performance of evaporator is extremely important to the overall cooling efficiency. In a well-designed evaporator, effective flow boiling heat transfer can be achieved whereas the critical heat flux (CHF) or dryout condition must be avoided. Otherwise the device surface temperature would rise significantly and cause device burnout due to the poor heat transfer performance of film boiling. In order to evaluate the influence of varying imposed heat fluxes, saturated flow boiling in the evaporator is systematically studied. The complete refrigerant flow boiling hysteresis between the imposed heat flux and the exit wall superheat is characterized. Upon the occurrence of CHF at the evaporator wall exit, the wall heat flux redistributes due to the axial wall heat conduction, which drives the dryout point to propagate upstream in the evaporator. As a result, a significant amount of thermal energy is stored in the evaporator wall. While the heat flux starts decreasing, the dryout point moves downstream and closer to the exit. The stored heat in the wall dissipates slowly and leads to the delay in rewetting or quenching, which is the key to understand and predict the flow boiling hysteresis. In order to reveal the transient heat releasing mechanism, an augmented separated-flow model is developed to predict the moving rewetting point and minimum heat flux at the evaporator exit, and the model predictions are further validated by experimental data from a refrigeration cooling testbed.

High Heat Flux With Small Scale Monodisperse Sprays

2012 ◽  
Vol 134 (12) ◽  
Author(s):  
Sergio Escobar-Vargas ◽  
Jorge E. Gonzalez ◽  
Drazen Fabris ◽  
Ratnesh Sharma ◽  
Cullen Bash
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Film Thickness ◽  
Liquid Film ◽  
Heat Dissipation ◽  
High Heat ◽  
Heat Fluxes ◽  
Liquid Film Thickness ◽  
Small Scale ◽  
High Heat Flux

This work is aimed at cooling small surfaces (1.3 mm × 2 mm and 3 mm × 5 mm) using spray from thermal ink jet (TIJ) atomizers. Particular interests in this work include obtaining heat fluxes near the critical heat flux (CHF), understanding the correlation between the heat dissipation efficiency (η) and the liquid film thickness (δ) through experimental data, and understanding the primary mode of heat transfer on spray cooling at different liquid film thickness. Current experimental results indicate that high heat fluxes (∼4 × 107 W/m2) are obtained for controlled conditions of cooling mass flow rate, higher efficiencies are achieved at smaller liquid film thickness (δ ≈ 5 μm → η ≈ 0.9), and the heat transfer by conduction through the film becomes dominant as δ decreases.

Local Heat Transfer of Saturated Flow Boiling in Vertical Narrow Microchannel

2020 ◽  
Vol 142 (7) ◽  
Author(s):  
Junye Li ◽  
Yuhao Lin ◽  
Wei Li ◽  
Kan Zhou
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Liquid Film ◽  
Annular Flow ◽  
Flow Boiling ◽  
High Heat ◽  
Flow Patterns ◽  
High Heat Flux ◽  
Bubble Flow ◽  
Saturated Flow

Abstract An experimental study of saturated flow boiling in a high-aspect-ratio one-side-heating rectangular microchannel was conducted with de-ionized water as the working fluid. ZnO microrods with the average diameter of about 1 μm and length of about 7 μm were synthesized on the Ti wafer surface, which was used to fabricate the heated bottom surface of the microchannel. The ZnO microrod surface appeared to be hydrophobic and the capillary wetting effect on the surface was found after being wet. The heat transfer and pressure drop characteristics of saturated flow boiling in the microchannel were studied and the flow patterns were photographed with a high-speed camera. Almost all the flow patterns observed in this experiment featured the main annular flow and abrupt flush of bubbly flow. Because of the capillary wetting effect on the ZnO microrod surface, the local dryout and rewetting phenomenon did not appear in this study. However, due to the numerous nucleation sites on ZnO microrod surface, the abrupt bubble flow caused much more disruption to the liquid film of annular flow when compared to the regular silicon surface. The abrupt bubble flow flushed through the annular liquid film and caused the fluctuation and nonuniformity of the liquid film and heat transfer deterioration, which was severer in the high heat flux conditions. Otherwise, the capillary effect on the ZnO microrod surface was able to restrict the nonuniformity of the liquid film under high heat flux and low mass flux conditions; thus, the deterioration of heat transfer performances diminished.

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