Author: Matthew A. Cryer Publisher: ISBN: 9781423547877 Category : Languages : en Pages : 61
Book Description
Recent studies have shown that thermophotovoltaic (TPV) technology is a promising source of high power density generation. Enhanced TPV systems can theoretically provide power densities of up to 100 W/cm2. The inherent ineffiencies in the system dictate that up to 90% of that energy is not converted to electrical power, and must be removed as waste heat to ensure that the components are maintained at a reasonable operating temperature. The present study addresses this issue by investigating the suitability of using spray cooling techniques to remove heat generated by power densities of up to 100 W/ cm2. A simple, scaleable experiment was designed using low cost commercially available components to study the effects that spray mass flux and droplet size have on the heat removal capacity of the system. A series of nozzles were used so that mass flux and droplet size could be studied independently, giving high resolution to the data so that predictive correlations could be developed over the range of parameters varied in the study.
Author: James B. Fillius Publisher: ISBN: 9781423521440 Category : Cooling Languages : en Pages : 79
Book Description
Spray cooling is a promising means of dissipating large steady state heat fluxes in high density power and electronic systems, such as thermophotovoltaic systems. The present study reports on the effectiveness of spray cooling in removing heat fluxes as high as 220 W/cm2. An experiment was designed to determine how the parameters of spray volumetric flow rate and droplet size influence the heat removal capacity of such a system. A series of commercially available nozzles were used to generate full cone water spray patterns encompassing a range of volumetric flow rates (3.79 to 42.32 L/h) and droplet Sauter mean diameters (17.4 to 35.5 micrometers). The non-flooded regime of spray cooling was studied, in which liquid spreading on the heater surface following droplet impact is the key phenomenon that determines the heat transfer rate. The experimental data established a direct proportionality of the heat flux with spray flow rate, and an inverse dependence on the droplet diameter. A correlation of the data was developed to predict heat flux as a function of the studied parameters over the range of values tested in this experiment.
Author: Alex M. Tulchinsky Publisher: ISBN: Category : Electronic apparatus and appliances Languages : en Pages : 306
Book Description
Spray impingement cooling has been shown to be a leading candidate for future high heat flux cooling applications. In general, spray cooling curves consist of three heat flux regimes; single-phase, two-phase and critical heat flux (CHF). CHF is considered the design limit for almost all two-phase cooling applications, as a rapid increase in temperature and decrease in heat flux occurs beyond this point. Recent studies have shown that the addition of micro-structures on the impingement surface can enhance heat transfer relative to a smooth surface. In the present study, spray cooling curves are obtained for two micro-structured surfaces and are compared to smooth surface results. Micro-structured surfaces consisted of bio-inspired fractal-like geometries, denoted as grooves and fins, extending in a radial direction from the center to the periphery of a 37.8 mm circular disc. Depending on the location on the surface, dimensions of groove widths and heights varied from 100 to 500 um, and 30 to 60 um, respectively. Fin width and height dimension remained constant throughout the surface at 127 and 60 um, respectively. Heat flux and wall temperature at the impingement surface were calculated from temperature data measured at multiple locations below the impingement surface. Results are presented as heat flux, q" , versus the wall-to-spray temperature difference, deltaT[subscript w], at each of 5 volume flux, Q", conditions ranging from 0.54 to 2.04 x 10−3 m3/m2s. Convection coefficients, h[subscript cv], and spray efficiencies, n, are also presented for each case as a function of q" and deltaT[subscript w] , respectively. Results of the study indicate that at low and high volume fluxes, an improvement in heat transfer occurs in the single-phase regime for the fin geometry. Enhancement in the single-phase regime did not occur at the intermediate volume flux condition of 1.37 x 10−3 m3/m2s. At all volume flux states tested, significant enhancements, as high as 50% in some cases, were observed in the two-phase regime for the fin structure, whereas the groove structure performed identically to the flat surface in the single-phase regime and exhibited a large degradation in the two-phase and critical heat flux regimes (~50%). Critical heat flux for the fin surface compared to the flat surface was slightly lower at low volume flux conditions, equivalent at the intermediate volume flux, and slightly greater at high volume flux conditions. Further investigations into the underlying mechanisms responsible for these results are needed.
Author: Guillermo Enrique Soriano Publisher: ISBN: Category : Languages : en Pages :
Book Description
Spray cooling is one of the most promising technologies in applications which require large heat removal capacity in very small areas. Previous experimental studies have suggested that one of the main mechanisms of heat removal in spray cooling is forced convection with strong mixing due to droplet impingement. These mechanisms have not been completely understood mainly due to the large number of physical variables, and the inability to modulate and control variables such as droplet frequency and droplet size. Our approach consists of minimizing the number of experimental variables by controlling variables such as droplet direction, velocity and diameter. A study of heat transfer for single and multiple droplet impingements using HFE- 7100 as the cooling fluid under constant heat flux conditions is presented. Monosized single and multiple droplet trains were produced using a piezoelectric droplet generator with the ability to adjust droplet frequency, diameter, velocity, and spacing between adjacent droplets. In this study, heaters consisting of a layer of Indium Tin Oxide (ITO) as heating element, and ZnSe substrates were used. Surface temperature at the liquid-solid interface was measured using Infrared Thermography. Heat transfer behavior was characterized and critical heat flux was measured. Film thickness was measured using a non-invasive optical technique inside the crown formation produced by the impinging droplets. Hydrodynamic phenomena at the droplet impact zone was studied using high speed imaging. Impact regimes of the impinging droplets were identified, and their effect on heat transfer performance were discussed. The results and effects of droplet frequency, droplet diameter, droplet velocity, and fluid flow rate on heat flux behavior, critical heat flux, and film morphology were elucidated. The study showed that forced heat convection is the main heat transfer mechanism inside the crown formation formed by droplet impingement and impact regimes play an important role on heat transfer behavior. In addition, this study found that spacing among adjacent droplets is the most important factor for multiple droplet stream heat transfer behavior. The knowledge generated through the study provides tools and know-how necessary for the design and development of enhanced spray cooling systems.
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.
Author: Warner A. McGhee Publisher: ISBN: Category : Languages : en Pages : 51
Book Description
Droplet cooling is used in many heat removal applications, including core spray coolers in boiling water reactors. As new accident tolerant fuels are developed, understanding how they respond to droplet cooling is important to ensuring safe operations. Recent studies have indicated that surfaces engineered with micro- and nanostructures may affect the Leidenfrost point temperature of water and thus the efficiency of droplet cooling by altering the wettability of the surfaces. In this project, smooth and rough chromium surfaces were subjected to droplet cooling at temperatures ranging from 100 to 400°C, and the surface temperature was measured with a high speed infrared camera while a video camera observed the droplet shape and behavior during boiling. While the rough and smooth surfaces performed similarly at temperatures below 200°C, the data indicates that at higher temperatures the smooth surface allows for greater heat flux, longer droplet contact time, and more total heat removed. The sparsity of data makes this result very uncertain, especially since it seems to oppose most literature on the topic. The techniques developed for this study are promising for future illumination how surface structure affects droplet cooling.
Author: S. M. Sohel Murshed Publisher: BoD – Books on Demand ISBN: 1789848385 Category : Science Languages : en Pages : 154
Book Description
Since conventional cooling techniques are increasing falling short of meeting the ever-growing cooling demands of high heat generating devices, thermal systems, and processes, advanced and innovative cooling technologies are of immense importance to deal with such high thermal management. Hence, this book covers a number of key topics related to advanced cooling approaches, their performance, and applications, including: Evaporative air cooling; Spray impingement cooling; Heat pump-based cooling; Modular cooling for photovoltaic plant; Nucleate pool boiling of refrigerants; Transient flashing spray cooling and application; Compressor cooling systems for industry. The book is aimed at a wide variety of people from graduate students and researchers to manufacturers who are involved or interested in the areas of thermal management systems, cooling technologies, and their applications.
Author: Matthew A. Cryer
Author: James B. Fillius
Author: Alex M. Tulchinsky
Author: Guillermo Enrique Soriano
Author: Andrea C. Ashwood
Author: Huseyin Bostanci
Author: Hamed Muhammed A. Al-Ahamdi
Author: John W. Schaefer
Author: Warner A. McGhee
Author: S. M. Sohel Murshed