An Experimental Study of a Bubble-Driven Heat-Transport Device

2007 ◽  
Author(s):  
Osamu Suzuki
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Heat Transport ◽  
Tube Bundle ◽  
Temperature Fluctuations ◽  
Temperature Gradients ◽  
Heating And Cooling ◽  
Tube Bundles ◽  
Steady State Time ◽  
Transport Device

We experimentally measured the heat-transport characteristics of a bubble-driven heat-transport device. The device consisted of a non-looped copper tube containing water. The tube was either meandered or spiraled to form tube bundles. The inner surface of the tube was smooth and its diameter small enough to enable the formation of vapor and liquid plugs in it. Two copper blocks were attached to the tube bundles, one as a heating block and the other as a cooling block. In the experiment, most of the wall temperatures measured on the tube fluctuated periodically at a quasi-steady state. Time-averaged temperature gradients between the heating and cooling sections of the device were constant. By increasing heater input from 300W to 350W, the amplitude of the temperature fluctuations decreased and the temperature gradients increased significantly. This behavior was regarded as a transition to critical heat transport condition. The effective thermal conductivity of the device was proportional to the heat-transport rate but did not depend on the formation of the tube bundle and the gravity effect. The temperature fluctuations had specific peak frequencies and a positive correlation was found between the frequency and effective thermal conductivity. These experimental results strongly suggest that the main heat-transport mechanism of the investigated device is based on the oscillation-induced transport of sensible heat.

Design Formulas for Oscillatory Heat Transport in Open-Ended Tubes

2003 ◽  
Vol 125 (6) ◽  
pp. 1183-1186 ◽  
Author(s):  
Masao Furukawa
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Heat Transport ◽  
Pipe Flow ◽  
Tube Bundle ◽  
Womersley Number ◽  
Algebraic Expressions ◽  
Conductivity Increase ◽  
Bundle Size ◽  
Relative Conductivity

Modified Watson’s functions dependent on the Womersley Number, concerning a forced oscillatory pipe flow, are introduced to mathematically simply express the effective thermal conductivity, the tidal displacement, and the tidal work of fluid. Those three are developed into algebraic expressions giving the required electrical oscillating power and the necessary number of capillary tubes. The relative conductivity increase, the specific shaker driving power, and the specific tube bundle size are graphically shown in the figures for several fluids of interest to contribute to designing a heaterless liquid warmer.

Steady-State Investigation of Vapor Deposited Micro Heat Pipe Arrays

1995 ◽  
Vol 117 (1) ◽  
pp. 75-81 ◽  
Author(s):  
A. K. Mallik ◽  
G. P. Peterson
Keyword(s):  
Thermal Conductivity ◽  
Steady State ◽  
Surface Temperature ◽  
Heat Pipe ◽  
Effective Thermal Conductivity ◽  
Heat Pipes ◽  
Temperature Gradients ◽  
Bottom Surface ◽  
Micro Heat Pipe ◽  
Micro Heat Pipes

An experimental investigation of vapor deposited micro heat pipe arrays was conducted using arrays of 34 and 66 micro heat pipes occupying 0.75 and 1.45 percent of the cross-sectional area, respectively. The performance of wafers containing the arrays was compared with that of a plain silicon wafer. All of the wafers had 8 × 8 mm thermofoil heaters located on the bottom surface to simulate the active devices in an actual application. The temperature distributions across the wafers were obtained using a Hughes Probeye TVS Infrared Thermal Imaging System and a standard VHS video recorder. For wafers containing arrays of 34 vapor deposited micro heat pipes, the steady-state experimental data indicated a reduction in the maximum surface temperature and temperature gradients of 24.4 and 27.4 percent, respectively, coupled with an improvement in the effective thermal conductivity of 41.7 percent. For wafers containing arrays of 66 vapor deposited micro heat pipes, the corresponding reductions in the surface temperature and temperature gradients were 29.0 and 41.7 percent, respectively, and the effective thermal conductivity increased 47.1 percent, for input heat fluxes of 4.70 W/cm2. The experimental results were compared with the results of a previously developed numerical model, which was shown to predict the temperature distribution with a high degree of accuracy, for wafers both with and without the heat pipe arrays.

scholarly journals A Study on Improving the Coefficient of Performance by Comparing Balancing Well and Standing Column Well Heat Exchange Systems

2020 ◽  
Vol 12 (24) ◽  
pp. 10445
Author(s):  
Myungkwan Lim ◽  
Kyoungbin Lim ◽  
Changhee Lee
Keyword(s):  
Thermal Conductivity ◽  
Heat Exchanger ◽  
Effective Thermal Conductivity ◽  
Water Discharge ◽  
Underground Water ◽  
Coefficient Of Performance ◽  
Geothermal System ◽  
Heat Accumulation ◽  
Heating And Cooling ◽  
Discharge Water

This study proposed a technology to improve the performance characteristics and coefficient of performance (COP) of a geothermal system by fundamentally preventing underground water discharge and maintaining a constant temperature of the underground heat exchanger composed of bleed discharge water that utilizes two balancing wells using cross-mixing methods. Using the standing column well (SCW) and cross-mixing balancing well underground heat exchanger, we compared and analyzed the effective thermal conductivity characteristics and COP characteristics during heating and cooling modes. Consequently, the cross-mixing balancing well underground heat exchanger exhibited more effective thermal conductivity than the SCW underground heat exchanger, with a high COP. Therefore, suggesting the performance was improved using groundwater flow rather than SCW. The comparison and analysis results of the effective heat map characteristics using the results of the SCW and balancing well system showed that the heating operation for the SCW underground heat exchanger had better thermal conductivity characteristics than the cooling operation. In addition, regarding a balancing well underground heat exchanger, the cooling operation exhibited superior thermal conductivity characteristics. Thus, the performance was considered to have improved due to the flow of activated groundwater in the ground and the rapid heat transfer without heat accumulation.

scholarly journals A macroscale mixture theory analysis of deposition and sublimation rates during heat and mass transfer in dry snow

2015 ◽  
Vol 9 (5) ◽  
pp. 1857-1878 ◽  
Author(s):  
A. C. Hansen ◽  
W. E. Foslien
Keyword(s):  
Thermal Conductivity ◽  
Mass Transfer ◽  
Diffusion Coefficient ◽  
Effective Thermal Conductivity ◽  
Heat And Mass Transfer ◽  
Effective Diffusion ◽  
Mixture Theory ◽  
Temperature Gradients ◽  
Snow Metamorphism ◽  
Sublimation Rates

Abstract. The microstructure of a dry alpine snowpack is a dynamic environment where microstructural evolution is driven by seasonal density profiles and weather conditions. Notably, temperature gradients on the order of 10–20 K m−1, or larger, are known to produce a faceted snow microstructure exhibiting little strength. However, while strong temperature gradients are widely accepted as the primary driver for kinetic growth, they do not fully account for the range of experimental observations. An additional factor influencing snow metamorphism is believed to be the rate of mass transfer at the macroscale. We develop a mixture theory capable of predicting macroscale deposition and/or sublimation in a snow cover under temperature gradient conditions. Temperature gradients and mass exchange are tracked over periods ranging from 1 to 10 days. Interesting heat and mass transfer behavior is observed near the ground, near the surface, as well as immediately above and below dense ice crusts. Information about deposition (condensation) and sublimation rates may help explain snow metamorphism phenomena that cannot be accounted for by temperature gradients alone. The macroscale heat and mass transfer analysis requires accurate representations of the effective thermal conductivity and the effective mass diffusion coefficient for snow. We develop analytical models for these parameters based on first principles at the microscale. The expressions derived contain no empirical adjustments, and further, provide self consistent values for effective thermal conductivity and the effective diffusion coefficient for the limiting cases of air and solid ice. The predicted values for these macroscale material parameters are also in excellent agreement with numerical results based on microscale finite element analyses of representative volume elements generated from X-ray tomography.

Experimental Investigation of a Three-Layer Oscillating Heat Pipe

2014 ◽  
Vol 136 (5) ◽  
Author(s):  
C. D. Smoot ◽  
H. B. Ma
Keyword(s):  
Thermal Conductivity ◽  
Experimental Investigation ◽  
Heat Pipe ◽  
Effective Thermal Conductivity ◽  
Heat Transport ◽  
Total Power ◽  
Oscillating Heat Pipe ◽  
Layer Effect ◽  
Layering Effect ◽  
Heat Transport Capability

An experimental investigation of a compact, triple-layer oscillating heat pipe (OHP) has been conducted to determine the channel layer effect on the heat transport capability in an OHP. The OHP has dimensions 13 mm thick, 229 mm long, and 76 mm wide embedded with two-independent closed loops forming three layers of channels. The unique design of the investigated OHP can be readily used to explore the channel layering effect on the heat transport capability in the OHP. The experimental results show that the addition of channel layers can increase the total power and at the same time, it can increase the effective thermal conductivity of the OHP. When the OHP switches from one layer of channels to two layers of channels, the highest effective thermal conductivity can be increased from 5760 W/mK to 26,560 W/mK. At the same time, the dryout limit can be increased. With three layers of channels, the OHP investigated herein can transport a power up to 8 kW with a heat flux level of 103 W/cm2 achieving an effective thermal conductivity of 33,170 W/mK.

Experimental Investigation of Micro Heat Pipes Fabricated in Silicon Wafers

1993 ◽  
Vol 115 (3) ◽  
pp. 751-756 ◽  
Author(s):  
G. P. Peterson ◽  
A. B. Duncan ◽  
M. H. Weichold
Keyword(s):  
Thermal Conductivity ◽  
Experimental Investigation ◽  
Heat Pipe ◽  
Effective Thermal Conductivity ◽  
Heat Pipes ◽  
Silicon Wafers ◽  
Temperature Gradients ◽  
Cover Plate ◽  
Micro Heat Pipe ◽  
Micro Heat Pipes

An experimental investigation was conducted to determine the thermal behavior of arrays of micro heat pipes fabricated in silicon wafers. Two types of micro heat pipe arrays were evaluated, one that utilized machined rectangular channels 45 μm wide and 80 μm deep and the other that used an anisotropic etching process to produce triangular channels 120 μm wide and 80 μm deep. Once fabricated, a clear pyrex cover plate was bonded to the top surface of each wafer using an ultraviolet bonding technique to form the micro heat pipe array. These micro heat pipe arrays were then evacuated and charged with a predetermined amount of methanol. Using an infrared thermal imaging unit, the temperature gradients and maximum localized temperatures were measured and an effective thermal conductivity was computed. The experimental results were compared with those obtained for a plain silicon wafer and indicated that incorporating an array of micro heat pipes as an integral part of semiconductor devices could significantly increase the effective thermal conductivity; decrease the temperature gradients occurring across the wafer; decrease the maximum wafer temperatures; and reduce the number and intensity of localized hot spots. At an input power of 4 W, reductions in the maximum chip temperature of 14.1°C and 24.9°C and increases in the effective thermal conductivity of 31 and 81 percent were measured for the machined rectangular and etched triangular heat pipe arrays, respectively. In addition to reducing the maximum wafer temperature and increasing the effective thermal conductivity, the incorporation of the micro heat pipe arrays was found to improve the transient thermal response of the silicon test wafers significantly.

scholarly journals Ballistic-Diffusive Model for Heat Transport in Superlattices and the Minimum Effective Heat Conductivity

Entropy ◽  
2020 ◽  
Vol 22 (2) ◽  
pp. 167 ◽  
Author(s):  
Federico Vázquez ◽  
Péter Ván ◽  
Róbert Kovács
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Heat Transport ◽  
Heat Conductivity ◽  
Room Temperature ◽  
Heat Fluxes ◽  
Multilayer Systems ◽  
Effective Heat Conductivity ◽  
Effective Heat ◽  
Kinetic Calculations

There has been much interest in semiconductor superlattices because of their low thermal conductivities. This makes them especially suitable for applications in a variety of devices for the thermoelectric generation of energy, heat control at the nanometric length scale, etc. Recent experiments have confirmed that the effective thermal conductivity of superlattices at room temperature have a minimum for very short periods (in the order of nanometers) as some kinetic calculations had anticipated previously. This work will show advances on a thermodynamic theory of heat transport in nanometric 1D multilayer systems by considering the separation of ballistic and diffusive heat fluxes, which are both described by Guyer-Krumhansl constitutive equations. The dispersion relations, as derived from the ballistic and diffusive heat transport equations, are used to derive an effective heat conductivity of the superlattice and to explain the minimum of the effective thermal conductivity.

Thermo-Fluid Model for High Thermal Conductivity Thermal Ground Planes

2012 ◽  
Author(s):  
Mohammed T. Ababneh ◽  
Frank M. Gerner ◽  
Pramod Chamarthy ◽  
Peter de Bock ◽  
Shakti Chauhan ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Gravitational Field ◽  
Effective Thermal Conductivity ◽  
Heat Transport ◽  
Fluid Model ◽  
Ground Plane ◽  
Maximum Heat ◽  
Gravitational Fields ◽  
Capillary Limit ◽  
Heat Transport Capability

The thermal ground plane (TGP) is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the TGP, including the effects of the presence of non-condensable gases (NCGs). Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the TGP decreases when the substrate and/or wick are thicker and/or with the presence of NCGs. Moreover, it was demonstrated that the thermo-fluid model may be utilized to optimize the performance of the TGP by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the TGP. The wick porosity plays an important effect on maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat transport capability of the TGP. Axial effective thermal conductivity is mostly unaffected by the gravitational field. The maximum length of the TGP before reaching the capillary limit is inversely proportional to input power.

Heat Transport in Evacuated Perlite Powders for Super-Insulated Long-Term Storages up to 300 °C

2013 ◽  
Vol 135 (5) ◽  
Author(s):  
Thomas Beikircher ◽  
Matthias Demharter
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Heat Transport ◽  
Solid Body ◽  
Storage Temperature ◽  
Hot Water ◽  
Cryogenic Temperatures ◽  
Real Size ◽  
Storage Temperatures

Vacuum super insulation (VSI) with expanded perlite powder is commonly used at cryogenic temperatures, but principally can also be adapted to applications at higher temperatures, such as the long-term storage of hot water in solar thermal systems. Due to the lack of experimental data in the respective temperature range, especially without external load, thermal conductivity measurements have been performed with commercial perlite powder up to 150°C mean sample temperature, corresponding to storage temperatures of around 300°C. Two different experimental geometries have been used: a guarded hot plate (GHP) setup and a cut-off concentric cylinder (CCC) apparatus. Furthermore, the radiative heat transport has been determined separately by extinction measurements using Fourier transform infrared (FTIR) spectroscopy. In addition to the laboratory experiments, a real-size prototype of a solar VSI-storage tank with 16.4 m3 water storage volume has been constructed, and the effective thermal conductivity of the perlite insulation has been determined from a heat loss measurement. The heat transport in evacuated perlite has also been treated theoretically using common models and approaches for gas heat conduction, solid-body conduction and heat transfer by thermal radiation. For the coupling between solid-body and gas conduction which occurs in the intergranular spaces of a powder material, a simple model has been developed. The total effective thermal conductivity λeff of a vacuum super insulation with dry, evacuated perlite powder (p≤0.01 mbar,ρ≈60 kg/m3) amounts to 0.007–0.016 W/mK for mean sample temperatures between 50°C and 150°C, compared to 0.003–0.005 W/mK at cryogenic temperatures. For the real-size storage prototype, the value λeff=0.009 W/mK has been obtained at T=90°C (storage temperature), p = 0.08 mbar and ρ=92.4 kg/m3, which compares to 0.03–0.06 W/mK for dry conventional storage insulations. With the applied theoretical models and approaches, the effective thermal conductivity of evacuated perlite and its individual contributions can successfully be described at different densities (55-95 kg/m3), compression methods, vacuum pressures (10-3-1000 mbar) and filling gases (air, Ar, Kr) up to mean sample temperatures of T=150°C. With regard to practical purposes, it has shown that vacuum super insulation with perlite is a suitable and economic method to achieve low thermal conductivities also at medium storage temperatures.

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