Boiling of Water at Sub-Atmospheric Conditions With Enhanced Structures

2005 ◽  
Author(s):  
Aniruddha Pal ◽  
Yogendra Joshi
Keyword(s):  
Heat Transfer ◽  
Electronics Cooling ◽  
High Heat ◽  
Maximum Temperature ◽  
Working Fluid ◽  
Atmospheric Conditions ◽  
Liquid Cooling ◽  
High Heat Transfer ◽  
Heat Loads ◽  
Very High

Liquid cooling with phase change is a very attractive option for thermal management of electronics because of the very high heat transfer coefficients achievable. Two-phase liquid cooling can be implemented in a thermosyphon loop, which has an evaporator, where heat is absorbed from the source during boiling of the working fluid, and a condenser, where the absorbed heat is rejected. Water is a preferred working fluid for boiling heat transfer due to its excellent thermal properties. Using water at sub-atmospheric conditions helps in initiation of boiling at low temperatures, which is necessary for electronics cooling applications, often limiting the maximum temperature to 85°C for silicon devices. Past studies have also shown that using boiling enhancement structures improve heat transfer by lowering the incipience overshoot, increasing heat flux and reducing evaporator volume. However, detailed study on the effects of enhancement structures and sub-atmospheric saturation conditions on the boiling of water in a compact thermosyphon loop is lacking in the literature. The objective of this study is to understand the boiling phenomena under the above-mentioned conditions and to investigate their effectiveness in electronics cooling applications. Experiments were carried out in a thermosyphon setup at 9.7, 15 and 21 kPa saturation pressures for two different enhancement structure geometries at varying heat loads (1–170 W). The experimental investigation showed that very high heat fluxes (≥ 80 W/cm2) can be achieved by boiling at sub-atmospheric pressures with enhancement structures. It is observed that with decreasing system pressure, the surface temperature also decreased for all the heat loads. The surface temperatures attained were well below the acceptable value of 85° C for all the cases.

Heat Transfer Characteristics of a Flow Passage With Long Pin Fins and Improving Heat Transfer Coefficient by Adding Turbulence Promoters on a Endwall

2001 ◽  
Author(s):  
K. Takeishi ◽  
T. Nakae ◽  
K. Watanabe ◽  
M. Hirayama
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Aspect Ratio ◽  
Transfer Coefficient ◽  
High Heat ◽  
Atmospheric Conditions ◽  
Pin Fin ◽  
High Heat Transfer ◽  
Turbine Vanes ◽  
Pin Fins

Pin fins are normally used for cooling the trailing edge region of a turbine, where their aspect ratio (height H/diameter D) is characteristically low. In small turbine vanes and blades, however, pin fins may also be located in the middle region of the airfoil. In this case, the aspect ratio can be quite large, usually obtaining values greater than 4. Heat transfer tests, which are conducted under atmospheric conditions for the cooling design of turbine vanes and blades, may overestimate the heat transfer coefficient of the pin-finned flow channel for such long pin fins. The fin efficiency of a long pin fin is almost unity in a low heat transfer situation as it would be encountered under atmospheric conditions, but can be considerably lower under high heat transfer conditions and for pin fins made of low thermal conductivity material. A series of tests with corresponding heat transfer models has been conducted in order to clarify the heat transfer characteristics of the long pin-finned flow channel. It is assumed that heat transfer coefficients can be predicted by the linear combination of two heat transfer equations, which were separately developed for the pin fin surface and for tubes in crossflow. To confirm the suggested combined equations, experiments have been carried out, in which the aspect ratio and the thermal conductivity of the pin were the test parameters. To maintain a high heat transfer coefficient for a long pin fin under high-pressure conditions, the heat transfer was augmented by adding a turbulence promoter on the pin-finned endwall surface. A corresponding equation that describes this situation has been developed. The predicted and measured values showed good agreement. In this paper, a comprehensive study on the heat transfer of a long pin-fin array will be presented.

Spatial and Temporal Wall Temperature Fluctuations in Two-Phase Flow in Microgap Coolers

2010 ◽  
Author(s):  
Jessica Sheehan ◽  
Avram Bar-Cohen
Keyword(s):  
Heat Transfer ◽  
High Heat ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Volumetric Cooling ◽  
Very High

Heat transfer to an evaporating refrigerant and/or dielectric liquid in a microgap channel can provide very high heat transfer coefficients and volumetric cooling rates. Recent studies at Maryland have established the dominance of the annular flow regime in such microgap channels and related the observed high-quality peak of an M-shaped heat transfer coefficient curve to the onset of local dryout. The present study utilizes infrared thermography to locate such nascent dryout regions and operating conditions. Data obtained with a 210 micron microgap channel, operated with a mass flux of 195.2 kg/m2-s and heat fluxes of 10.3 to 26 W/cm2 are presented and discussed.

Heat Pipe Solar Receivers for Concentrating Solar Power (CSP) Plants

2013 ◽  
Author(s):  
Yiding Cao
Keyword(s):  
Heat Transfer ◽  
Heat Pipe ◽  
Solar Power ◽  
High Heat ◽  
Solar Flux ◽  
Heat Transfer Fluid ◽  
Working Fluid ◽  
Efficient Operation ◽  
High Heat Transfer ◽  
Solar Receiver

This paper introduces separate-type heat pipe (STHP) based solar receiver systems that enable more efficient operation of concentrated solar power plants without relying on a heat transfer fluid. The solar receiver system may consist of a number of STHP modules that receive concentrated solar flux from a solar collector system, spread the high concentrated solar flux to a low heat flux level, and effectively transfer the received heat to the working fluid of a heat engine to enable a higher working temperature and higher plant efficiency. In general, the introduced STHP solar receiver has characteristics of high heat transfer capacity, high heat transfer coefficient in the evaporator to handle a high concentrated solar flux, non-condensable gas release mechanism, and lower costs. The STHP receiver in a solar plant may also integrate the hot/cold tank based thermal energy storage system without using a heat transfer fluid.

Experimental Investigation of Heat Transfer Augmentation by Different Jet Impingement Hole Shapes Under Maximum Crossflow

2016 ◽  
Author(s):  
Prashant Singh ◽  
Bharath Viswanath Ravi ◽  
Srinath Ekkad
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Jet Impingement ◽  
High Heat ◽  
Absolute Pressure ◽  
Target Plate ◽  
Heat Transfer Augmentation ◽  
Plate Spacing ◽  
High Heat Transfer ◽  
Heat Loads

To achieve higher overall efficiency in gas turbine engines, hot gas path components are subjected to high heat transfer loads due to higher turbine inlet temperatures. Jet impingement has been extensively used especially as an internal cooling technique in the leading edge and mid-chord region of first stage vanes, which are subjected to highest heat loads. With the advent of additive manufacturing methods such as Direct Metal Laser Sintering (DMLS), designers are not limited to designing round or race track holes for impingement. The present study is focused on exploring new jet hole shapes, in an arrangement, typical of mid-chord region in a double wall cooling configuration. Transient liquid crystal experiments are carried out to study heat transfer augmentation by jet impingement on smooth target where the spent air is allowed to exit in one direction, thus imposing maximum crossflow condition. The averaged Reynolds number (based on jet hydraulic diameter) is varied from 2500 to 10000. The jet plate has a square array of jets with 7 jets in one row (total number of jets = 49), featuring hole shapes — Racetrack and V, where the baseline case is the round hole. The non-dimensional streamwise (x/dj) and spanwise (y/dj) spacing is 6 and the normalized jet-to-target-plate spacing (z/dj) is 4 and the nozzle aspect ratio (L/dj) is also 4. The criteria for the hole shape design was to keep the effective area of different hole shapes to be the same, which resulted in slightly different hydraulic diameters. The jet-to-target plate spacing (z) has been adjusted accordingly so as to maintain a uniform z/dj of 4, across all three configurations studied. Heat transfer coefficients are measured using a transient Liquid Crystal technique employing a one-dimensional semi-infinite model. Flow experiments are carried out to measure static pressures in the plenum chamber, to calculate the discharge coefficient, for a range of plenum absolute pressure-to-ambient pressure ratios. Detailed normalized Nusselt number contours have been presented, to identify the regions of high heat transfer augmentation locally, so as to help the designers in the organization of jet hole shapes and their patterns in an airfoil depending upon the active heat loads.

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.

Microchannels and Minichannels: History, Terminology, Classification and Current Research Needs

2003 ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Historical Perspective ◽  
High Heat ◽  
High Flux ◽  
Research Needs ◽  
Transfer Rates ◽  
High Heat Transfer ◽  
Liver And Kidney ◽  
The Brain ◽  
Very High

Microchannels and Minichannels are found in many biological systems providing very high heat and mass transfer rates in organs such as the brain, lung, liver and kidney. Many high flux cooling applications are effectively utilizing their high heat transfer capabilities of these channels. A brief overview of the historical perspective and some of the issues that need to be addressed with microchannels and Minichannels are presented in this paper.

Experimental and Analytical Studies of Reciprocating-Mechanism Driven Heat Loops (RMDHLs)

2008 ◽  
Vol 130 (7) ◽  
Author(s):  
Yiding Cao ◽  
Mingcong Gao
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Electronics Cooling ◽  
High Heat ◽  
Working Fluid ◽  
High Heat Flux ◽  
Two Phase ◽  
Condenser Section ◽  
Analytical Studies ◽  
Evaporator Section

This paper conducts experimental and analytical studies of a novel heat-transfer device, reciprocating-mechanism driven heat loop (RMDHL) that facilitates two-phase heat transfer while eliminating the so-called cavitation problem commonly encountered by a conventional pump. A RMDHL normally includes a hollow loop having an interior flow passage, an amount of working fluid filled within the loop, and a reciprocating driver. The hollow loop has an evaporator section, a condenser section, and a liquid reservoir. The reciprocating driver is integrated with the liquid reservoir and facilitates a reciprocating flow of the working fluid within the loop, so that liquid is supplied from the condenser section to the evaporator section under a substantially saturated condition and the so-called cavitation problem associated with a conventional pump is avoided. The reciprocating driver could be a solenoid-operated reciprocating driver for electronics cooling applications and a bellows-type reciprocating driver for high-temperature applications. Experimental study has been undertaken for a solenoid-operated heat loop in connection with high heat flux thermal management applications. Experimental results show that the heat loop worked very effectively and a heat flux as high as 300W∕cm2 in the evaporator section could be handled. A working criterion has also been derived, which could provide a guidance for the design of a RMDHL.

Conduction through Droplets during Dropwise Condensation

1973 ◽  
Vol 95 (1) ◽  
pp. 12-19 ◽  
Author(s):  
Charles J. Hurst ◽  
Donald R. Olson
Keyword(s):  
Heat Transfer ◽  
Finite Element Analysis ◽  
Experimental Investigation ◽  
High Heat ◽  
Dropwise Condensation ◽  
Lower Side ◽  
Element Analysis ◽  
High Heat Transfer ◽  
Very High ◽  
Good Agreement

An experimental investigation was undertaken in which dropwise condensation was caused to occur on the upper side of a 0.001-in-thick horizontal copper condensing surface. The lower side of the condensing wall was convectively cooled, and the cooled-side temperatures under growing droplets were measured using infrared-radiation techniques. Temperature measurements showed good agreement with the results of a finite-element analysis of the droplet and condensing surface. Both experimental and analytical results pointed to the existence of an area of very high heat transfer right around the droplet perimeter, and to the importance of the condensing wall as a heat-diffusing mechanism in dropwise condensation.

Two-Phase Spray Cooling With Water/2-Propanol Binary Mixture: Investigation of Mass Diffusion Resistance

2016 ◽  
Author(s):  
Sai Sujith Obuladinne ◽  
Huseyin Bostanci
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Performance ◽  
Pure Water ◽  
Spray Cooling ◽  
High Heat ◽  
Working Fluid ◽  
High Heat Flux ◽  
Two Phase ◽  
High Heat Transfer

Two-phase spray cooling has been an emerging thermal management technique offering high heat transfer coefficients (HTCs) and critical heat flux (CHF) levels, near-uniform surface temperatures, and efficient coolant usage that enables to design of compact and lightweight systems. Due to these capabilities, spray cooling is a promising approach for high heat flux applications in computing, power electronics, and optics. The two-phase spray cooling inherently depends on saturation temperature-pressure relationships of the working fluid to take advantage of high heat transfer rates associated with liquid-vapor phase change. When a certain application requires strict temperature and/or pressure conditions, thermophysical properties of the working fluid play a critical role in attaining proper efficiency, reliability, or packaging structure. However, some of the commonly used working fluids today, including refrigerants and dielectric liquids, have relatively poor properties and heat transfer performance. In such cases, utilizing binary mixtures to tune working fluid properties becomes an alternative approach. This study aimed to conduct an initial investigation on the spray cooling characteristics of practically important binary mixtures and demonstrate their capability for challenging high heat flux applications. The working fluid, water/2-propanol binary mixture at various concentration levels, specifically at x1 (liquid mass fraction of 2-proponal in water) of 0.0 (pure water), 0.25, 0.50, 0.879 (azeotropic mixture) and 1.0, represented both non-azeotropic and azeotropic cases. Tests were performed on a closed loop spray cooling system using a pressure atomized spray nozzle with a constant liquid flow rate at corresponding 20°C subcooling conditions and 1 Atm pressure. A copper test section measuring 10 mm × 10 mm × 2 mm with a plain, smooth surface simulated high heat flux source. Experimental procedure involved controlling the heat flux in increasing steps, and recording the steady-state temperatures to obtain cooling curves in the form of surface superheat vs heat flux. The obtained results showed that pure water (x1 = 0.0) and 2-propanol (x1 = 1.0) provide the highest and lowest heat transfer performance, respectively. At a given heat flux level, the HTC values indicated strong dependence on x1, where the HTCs depress proportional to the concentration difference between the liquid and vapor phases. The CHF values sharply decreased at x1≥ 0.25.

Effect of Geometrical Parameters on Flow Mal-Distribution in a Wavy Microchannel

2015 ◽  
Vol 813-814 ◽  
pp. 674-678
Author(s):  
M. Satheeshkumar ◽  
M.R. Thansekhar ◽  
C. Anbumeenakshi ◽  
S. Suresh
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Flow Distribution ◽  
High Heat ◽  
Geometrical Parameters ◽  
Transfer Coefficients ◽  
Geometrical Properties ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Very High

Microchannels are of current interest for use in heat exchangers, where very high heat transfer performance is desired. Microchannels provide very high heat transfer coefficients because of their small hydraulic diameters. In this study, a numerical investigation of fluid flow in microchannels with varying hydraulic diameters is presented. Six channels with wavy shape are considered. Header is the major part in the microchannel, which supplies fluid into different channels. A CFD model was created to simulate the fluid flow in the header and microchannels. In this work, five different shapes of the header were considered namely circular, frustum conical, rectangular, triangular and trapezoidal. The results from these simulations are presented, and it is observed that the flow distribution is significantly affected by geometrical properties of the channel and the header.

Sign in / Sign up

Export Citation Format

Share Document