Pulse Mitigation and Heat Transfer Enhancement Techniques. Volume 1. Spray Cooling PDF Download
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Author: Publisher: ISBN: Category : Languages : en Pages : 95
Book Description
This report presents an experimental and theoretical investigation of spray cooling. A complete study is presented for surfaces maintained between the fluid saturation temperature and the Leidenfrost temperature. Experiments were conducted to analyze the effects of spray and surface conditions on the heat transfer. Different mechanisms causing the critical heat flux (CHF) are identified. For dropwise evaporation, CHF results when the surface heat flux exceeds the latent heat content of the spray. As the flow rate is increased, droplet conglomeration and surface flooding result. For low flow rate cases with a flooded surface, CHF is caused by a liquid deficiency resulting from droplet expulsion caused by the nucleating bubbles within the liquid film. For higher flow rate cases, CHF occurs when the vapor generation rate on the surface is so high that a vapor barrier is formed.
Author: Publisher: ISBN: Category : Languages : en Pages : 95
Book Description
This report presents an experimental and theoretical investigation of spray cooling. A complete study is presented for surfaces maintained between the fluid saturation temperature and the Leidenfrost temperature. Experiments were conducted to analyze the effects of spray and surface conditions on the heat transfer. Different mechanisms causing the critical heat flux (CHF) are identified. For dropwise evaporation, CHF results when the surface heat flux exceeds the latent heat content of the spray. As the flow rate is increased, droplet conglomeration and surface flooding result. For low flow rate cases with a flooded surface, CHF is caused by a liquid deficiency resulting from droplet expulsion caused by the nucleating bubbles within the liquid film. For higher flow rate cases, CHF occurs when the vapor generation rate on the surface is so high that a vapor barrier is formed.
Author: Publisher: ISBN: Category : Languages : en Pages : 141
Book Description
In the spray cooling of a heat surface, variations in the surface contact angle cause a change in nucleation characteristics and thereby influence the heat transfer process. The surface roughness variations affect the thickness of the liquid film formed by the spray; this has a profound effect on the heat transfer from the surface. The effect of these two parameters is studied experimentally under different spray conditions involving an air atomizing nozzle.
Author: World Bank Publisher: World Bank Publications ISBN: 9780821323953 Category : Business & Economics Languages : en Pages : 1422
Book Description
The Republic of Korea's industrial policy has directed that nation's economy through nearly three decades of spectacular growth. But the authors of this paper maintain that this policy is showing signs of being outmoded. The time has come, the authors argue, for the Korean government to stop managing the economy's structural development and to redefine the responsibilities of business and government. Under this proposed compact, the allocation of resources would shift from the government to the private industrial and financial sectors. The transformation of the government bureaucracy from an ad hoc policy role to one of a transparent and predictable regulator is a key to the success of this undertaking. These new directions would present the government with enormous challenges. Greater competitive discipline and regulatory oversight would be required. While dealing with the complexities of the transition, the government would have to maintain macroeconomic stability and the momentum of savings and investment. For comparison, the study examines the industrial economies of France, Germany, Japan, and the United States, which underwent similar shifts.
Author: Christian David Martinez Publisher: ISBN: Category : Languages : en Pages :
Book Description
ABSTRACT: Spray cooling is a technique for achieving large heat fluxes at low surface temperatures by impinging a liquid in droplet form on a heated surface. Heat is removed by droplets spreading across the surface, thus removing heat by evaporation and by an increase in the convective heat transfer coefficient. The addition of nano-sized particles, like aluminum or copper, to water to create a nanofluid could further enhance the spray cooling process. Nanofluids have been shown to have better thermophysical properties when compared to water, like enhanced thermal conductivity. Although droplet size, velocity, impact angle and the roughness of the heated surface are all factors that determine the amount of heat that can be removed, the dominant driving mechanism for heat dissipation by spray cooling is difficult to determine. In the current study, experiments were conducted to compare the enhancement to heat transfer caused by using alumina nanofluids during spray cooling instead of de-ionized water for the same nozzle pressure and distance from the heated surface. The fluids were sprayed on a heated copper surface at a constant distance of 21 mm. Three mass concentrations, 0.1%, 0.5%, and 1.0%, of alumina nanofluids were compared against water at three pressures, 40psi, 45psi, and 50psi. To ensure the suspension of the aluminum oxide nanoparticles during the experiment, the pH level of the nanofluid was altered. The nanofluids showed an enhancement during the single-phase heat transfer and an increase in the critical heat flux (CHF). The spray cooling heat transfer curve shifted to the right for all concentrations investigated, indicating a delay in two-phase heat transfer. The surface roughness of the copper surface was measured before and after spray cooling as a possible cause for the delay.
Author: Huseyin Bostanci Publisher: ISBN: Category : Ammonia Languages : en Pages : 110
Book Description
Many critical applications today, in electronics, optics and aerospace fields, among others, demand advanced thermal management solutions for the acquisition of high heat loads they generate in order to operate reliably and efficiently. Current competing technologies for this challenging task include several single and two phase cooling options. When these cooling schemes are compared based on the high heat flux removal (100-1000 W/cm2) and isothermal operation (within several °C across the cooled device) aspects, as well as system mass, volume and power consumption, spray cooling appears to be the best choice. The current study focused on high heat flux spray cooling with ammonia on enhanced surfaces. Compared to some other commonly used coolants, ammonia possesses important advantages such as low saturation temperature, and high heat absorbing capability. Moreover, enhanced surfaces offer potential to greatly improve heat transfer performance. The main objectives of the study were to investigate the effect of surface enhancement on spray cooling performance, and contribute to the current understanding of spray cooling heat transfer mechanisms. These objectives were pursued through a two stage experimental study. While the first stage investigated enhanced surfaces for the highest heat transfer coefficient at heat fluxes of up to 500 W/cm2, the second stage investigated the optimized enhanced surfaces for critical heat flux (CHF). Surface modification techniques were utilized to obtain micro scale indentations and protrusions, and macro (mm) scale pyramidal, triangular, rectangular, and square pin fins. A third group, multi-scale structured surfaces, combined macro and micro scale structures. Experimental results indicated that micro- and macrostructured surfaces can provide heat transfer coefficients of up to 534,000 and 426,000 W/m2°C at 500 W/cm2, respectively. Multi-scale structured surfaces offered even a better performance, with heat transfer coefficients of up to 772,000 W/m2°C at 500 W/cm2, corresponding to a 161% increase over the reference smooth surface. In CHF tests, the optimized multi-scale structured surface helped increase maximum heat flux limit by 18%, to 910 W/cm2 at nominal liquid flow rate. During the additional CHF testing at higher flow rates, most heaters experienced failures before reaching CHF at heat fluxes above 950 W/cm2. However, the effect of flow rate was still characterized, suggesting that enhanced surfaces can achieve CHF values of up to [almost equal to]1,100 W/cm2 with [almost equal to]67% spray cooling efficiency. The results also helped shed some light on the current understanding of the spray cooling heat transfer mechanisms. Data clearly proved that in addition to fairly well established mechanisms of forced convection in the single phase regime, and free surface evaporation and boiling through secondary nucleation in the two phase regime, enhanced surfaces can substantially improve boiling through surface nucleation, which can also be supported by the concept of three phase contact lines, the regions where solid, liquid and vapor phases meet. Furthermore, enhanced surfaces are capable of retaining more liquid compared to a smooth surface, and efficiently spread the liquid film via capillary force within the structures. This unique advantage delays the occurrence of dry patches at high heat fluxes, and leads to higher CHF.
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Author: World Bank
Author: Johnathan Stuart Coursey
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Author: Christian David Martinez
Author: K Chang
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Author: Huseyin Bostanci