Effect of Jet Arrays and Structured Surfaces on Two-Phase Impingement Heat Transfer

2007 ◽  
Author(s):  
Xin Feng ◽  
James E. Bryan
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Performance ◽  
Jet Impingement ◽  
Heat Fluxes ◽  
Transfer Efficiency ◽  
Surface Wetting ◽  
Two Phase ◽  
Structured Surfaces ◽  
Heat Transfer Efficiency ◽  
Impingement Heat Transfer

When heat fluxes and heat transport exceed 100 W/cm2, heat transfer efficiencies decrease rapidly. Experimental work will be presented exploring how micro jet arrays and structured surfaces can be used to increase heat transfer efficiency. Using water, ethanol, and HFE-7000 as working fluids, the effect of jet momentum, subcooling temperature and surface wetting are experimentally investigated on 1cm2 smooth and structured surfaces. From results obtained so far, heat transfer efficiency increases with increasing surface tension (decreasing surface wetting) with micro-jet arrays. Further, existing correlations for two-phase jet impingement cannot predict the heat transfer performance with acceptable accuracy as they do not account for surface wetting characteristics.

Experimental Study on the Thermal Properties and Stability of Hybrid Nanofluids and Evaluation of its Heat Exchange Efficiency

2020 ◽  
Author(s):  
Yubai Xiao ◽  
Hu Zhang ◽  
Junmei Wu
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Transfer Performance ◽  
Volume Concentration ◽  
High Thermal Conductivity ◽  
Transfer Efficiency ◽  
Working Fluid ◽  
Transfer Performance ◽  
Heat Transfer Efficiency ◽  
Hybrid Nanofluids

Abstract In recent years, hybrid nanofluids, as a new kind of working fluid, have been widely studied because they possessing better heat transfer performance than single component nanofluids when prepared with proper constituents and proportions. The application of hybrid nanofluids in nuclear power system as a working fluid is an effective way of improving the capability of In-Vessel Retention (IVR) when the reactor is in a severe accident. In order to obtain hybrid nanofluids with excellent heat transfer performance, three kinds of hybrid nanofluids with high thermal conductivity are measured by transient plane source method, and their viscosity and stability are also investigated experimentally. These experimental results are used to evaluate the heat transfer efficiency of hybrid nanofluids. The results show that: (1) The thermal conductivity of hybrid nanofluids increases with increasing temperature and volume concentration. When compared to the base fluid, the thermal conductivity of Al2O3-CuO/H2O, Al2O3-C/H2O and AlN-TiO2/H2O nanofluids at 0.25% volume concentration increased by 36%, 24%, and 22%, respectively. (2) Surfactants can improve the stability of hybrid nanofluids. The Zeta potential value is related to the thermal conductivity of the hybrid nanofluids, and it could be used to explain the relationship between the thermal conductivity of the hybrid nanofluids and the dispersion. It also could provide a reference for subsequent screening of high thermal conductivity nanofluids. (3) The addition of C/H2O can effectively reduce the dynamic viscosity coefficient of hybrid nanofluids. (4) The analysis of heat transfer efficiency of the hybrid nanofluids found that both Al2O3-CuO/H2O and Al2O3-C/H2O have better heat transfer ability than water under certain mixing conditions. This study is conducive to further optimizing hybrid nanofluids and its application to the In-Vessel Retention in severe reactor accidents.

Experimental Investigation of Area Ratio on Microjet Array Heat Transfer

2009 ◽  
Author(s):  
Gregory J. Michna ◽  
Eric A. Browne ◽  
Yoav Peles ◽  
Michael K. Jensen
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Performance ◽  
Electronics Cooling ◽  
Jet Impingement ◽  
Area Ratio ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Transfer Performance ◽  
Transfer Coefficients ◽  
High Heat Transfer

Electronics cooling is becoming increasingly difficult due to increasing power consumption and decreasing size of processor chips. Heat fluxes in processors and power electronics are quickly approaching levels that cannot be easily addressed by forced air convection over finned heat sinks. Jet impingement cooling offers high heat transfer coefficients and has been used effectively in conventional-scale applications such as turbine blade cooling and the quenching of metals. However, literature in the area of microjet arrays is scarce and has not studied arrays of large area ratios. Hence, the objective of this study is to experimentally assess the heat transfer performance of arrays of microjets. The microjet arrays were fabricated using MEMS processes in a clean room environment. The heat transfer performance of several arrays using deionized water as the working fluid was investigated. Inline and staggered array arrangements were investigated, and the area ratio (total area of the jets divided by the surface area) was varied between 0.036 and 0.35. Reynolds numbers defined by the jet diameter were in the range of 50 to 3,500. Heat fluxes greater than 1,000 W/cm2 were obtained at fluid inlet-to-surface temperature differences of less than 30 °C. Heat transfer performance improved as the area ratio was increased.

Effect of Electric Fields on Two-Phase Impingement Heat Transfer

2007 ◽  
Author(s):  
Xin Feng ◽  
James E. Bryan
Keyword(s):  
Heat Transfer ◽  
Electric Fields ◽  
Capillary Flow ◽  
Heated Surface ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Geometrical Parameters ◽  
Two Phase ◽  
Transfer Rates ◽  
Impingement Heat Transfer

The effect of electric fields applied to two-phase impingement heat transfer is explored for the first time. The application of an electric field between a capillary and heated surface results in the ability to control the free boundary flow from discreet drops to jets to sprays. Through an experimental study, the impingement heat transfer was evaluated over a range of operating and geometrical parameters using subcooled ethanol as the working fluid. The ability to change the mode of impinging mass did change the surface heat transfer. The characteristics of the impinging mass on heat transfer was dependent on capillary flow rate, applied voltage, capillary to heated surface spacing, capillary geometry, and heat flux. Enhancement occurred primarily at low heat fluxes (below 30 W/cm2) under ramified spray conditions where the droplet momentum promoted thin films on the heated surface. Higher heat fluxes resulted in greater vapor momentum from the surface minimizing the effect of different modes. However, under ramified spray conditions less mass was impacting the heated surface showing that heat transfer rates at higher heat fluxes were achievable with less mass, resulting in greater evaporation efficiency.

Effects of a Vortex Generator Pair on Jet Impingement Heat Transfer in Cross-Flow

2015 ◽  
Author(s):  
Chenglong Wang ◽  
Lei Wang ◽  
Bengt Sundén ◽  
Johan Revstedt
Keyword(s):  
Heat Transfer ◽  
Gas Turbines ◽  
Heat Transfer Performance ◽  
Cross Flow ◽  
Vortex Generator ◽  
Jet Impingement ◽  
Transfer Performance ◽  
Stagnation Region ◽  
Impingement Heat Transfer ◽  
Jet Nozzle

Jet impingement cooling is commonly used in gas turbines. Usually the spent air from the upstream jets forms a cross-flow past the downstream jets, which degrades their heat transfer performance. In the present study, a new method was proposed to promote the jet penetration and enhance the impingement heat transfer. By placing a delta-winglet vortex generator pair (VGP) in the cross-flow upstream of the jet nozzle, it is found that the impingement heat transfer on the target wall is significantly enhanced. The stagnation region shifts upstream and expands compared to the original case. The stagnation and area-averaged Nusselt numbers also increased. The effects of the distance between the VGP and the jet nozzle l1 were also investigated. The optimal spacing l1 is suggested to be 4d, giving the best heat transfer performance. This study sheds new light on the enhancement of jet impingement heat transfer in a cross-flow.

scholarly journals Study on Heat Transfer Performance and Anti-Fouling Mechanism of Ternary Ni-W-P Coating

2020 ◽  
Vol 10 (11) ◽  
pp. 3905
Author(s):  
Lu Ren ◽  
Yanhai Cheng ◽  
Jinyong Yang ◽  
Qingguo Wang
Keyword(s):  
Heat Transfer ◽  
Surface Energy ◽  
Transition State ◽  
Heat Transfer Performance ◽  
High Surface ◽  
Deposition Process ◽  
Oxidation Products ◽  
Transfer Efficiency ◽  
Transfer Performance ◽  
Heat Transfer Efficiency

Since the formation of fouling reduces heat transfer efficiency and causes energy loss, anti-fouling is desirable and may be achieved by coating. In this work, a nickel-tungsten-phosphorus (Ni-W-P) coating was prepared on the mild steel (1015) substrate using electroless plating by varying sodium tungstate concentration to improve its anti-fouling property. Surface morphology, microstructure, fouling behavior, and heat transfer performance of coatings were further reported. Also, the reaction path, transition state, and energy gradient change of calcite, aragonite, and vaterite were also calculated. During the deposition process, as the W and P elements were solids dissolved in the Ni crystal cell, the content of Ni element was obviously higher than that of the other two elements. Globular morphology was evenly covered on the surface. Consequently, the thermal conductivity of ternary Ni-W-P coating decreases from 8.48 W/m·K to 8.19 W/m·K with the increase of W content. Additionally, it goes up to 8.93 W/m·K with the increase of heat source temperature 343 K. Oxidation products are always accompanied by deposits of calcite-phase CaCO3 fouling. Due to the low surface energy of Ni-W-P coating, Ca2+ and [CO3]2− are prone to cross the transition state with a low energy barrier of 0.10 eV, resulting in the more formation of aragonite-phase CaCO3 fouling on ternary Ni-W-P coating. Nevertheless, because of the interaction of high surface energy and oxidation products on the bare matrix or Ni-W-P coating with superior W content, free Ca2+ and [CO3]2− can be easy to nucleate into calcite. As time goes on, the heat transfer efficiency of material with Ni-W-P coating is superior to the bare surface.

scholarly journals Influence of Geometrical Changes in an Adiabatic Portion on the Heat Transfer Performance of a Two-Phase Closed Thermosiphon System

Energies ◽  
2021 ◽  
Vol 14 (11) ◽  
pp. 3070
Author(s):  
Mohanraj Chandran ◽  
Rajvikram Madurai Elavarasan ◽  
Ramesh Babu Neelakandan ◽  
Umashankar Subramaniam ◽  
Rishi Pugazhendhi
Keyword(s):  
Heat Transfer ◽  
Thermal Resistance ◽  
Heat Transfer Performance ◽  
Heat Fluxes ◽  
Heat Transfer Analysis ◽  
Transfer Performance ◽  
Two Phase ◽  
Cross Sectional ◽  
Inclination Angles ◽  
Adiabatic Section

In this study, a modified non-uniform adiabatic section in a Two-Phase Closed Thermosiphon (TPCT) is proposed where the uniform section was replaced by convergent and divergent (C-D) sections. The heat transfer analysis was performed on the modified TPCT and their findings were compared with standard TPCT. The deionized water (DI) in the proportion of 30 vol% is filled in both the TPCTs. Further, the heat transfer performance analysis was carried out for three different orientations, such as 0°, 45° and 90°, and heat input was varied from 50 to 250 W. The effect of these geometrical changes and inclination angles on the heat transfer performance of both the TPCT were evaluated to compare the thermal resistance, wall temperature variation and heat transfer coefficient. The non-dimensional numbers such as Weber (WE), Bond (BO), Condensation (CO) and Kutateladze (KU) were investigated based on heat fluxes for both TPCTs. By introducing the convergent-divergent section nearer to the condenser, the pressure before and after the C-D section was increased and decreased. This enhances the heat transfer in the evaporator slightly up to 2% and 1.4% at horizontal and 45° orientation, respectively, in Non-Uniformed Adiabatic Section (NUAS) TPCT when compared to Uniformed Adiabatic Section (UAS) TPCT. The thermal resistance of NUAS TPCT was reduced by up to 4.5% relative to UAS TPCT in horizontal and 45°. The results of the non-dimensional number also confirmed that NUAS TPCT provided better performance by enhancing 2% more pool boiling characteristics, interaction forces and condensate returns. Several factors such as gravity assistance, fluid accumulation, pressure drop and thermal resistance exert an influence on the heat transfer performance of the proposed NUAS TPCT at various orientation angles. However, different type of cross-sectional variations subjected to orientation changes may also get influenced by several other parameters that in turn affect the heat transfer performance distinctly.

Application of Electrohydrodynamic Atomization to Two-Phase Impingement Heat Transfer

2008 ◽  
Vol 130 (7) ◽  
Author(s):  
Xin Feng ◽  
James E. Bryan
Keyword(s):  
Heat Transfer ◽  
Capillary Tube ◽  
Heated Surface ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
Working Fluid ◽  
Heater Surface ◽  
Two Phase ◽  
Impingement Heat Transfer ◽  
Repulsive Forces

The effect of electric fields applied to two-phase impingement heat transfer is explored for the first time. The electric field applied between a capillary tube and heated surface enhances the heat transfer by controlling the free boundary flow modes from discreet drops to jets, to sprays. Through an experimental study, the impingement heat transfer was evaluated over a range of operating conditions and geometrical parameters with subcooled ethanol used as the working fluid. The ability to change the mode of impinging mass did change the surface heat transfer. The characteristics of the impinging mass on heat transfer were dependent on flow rate, applied voltage, capillary tube to heated surface spacing, capillary tube geometry, heat flux, heater surface geometry, and capillary tube array configuration. Enhancement occurred primarily at low heat fluxes (below 30W∕cm2) under ramified spray conditions where the droplet momentum promoted thin films on the heated surface resulting in 1.7 times enhancement under certain conditions. Higher heat fluxes resulted in greater vapor momentum from the surface, minimizing the effect of different impingement modes. The use of capillary tube array allowed for electrohydrodynamics atomization enhancement and higher liquid flow rates, but electrostatic repulsive forces diverted the spray from the heater surface. This reduced the mass flux to the surface, leading to premature dryout under certain conditions.

Sign in / Sign up

Export Citation Format

Share Document