Large Temperature Drop Research Articles

An experimental investigation of the thermal performance of an asymmetrical flat plate heat pipe

An experimental investigation of the thermal performance of a flat plate heat pipe is presented in this work. The results indicate that the temperature along the heat pipe wall surfaces is quite uniform. The results also indicate that the porous wick of the evaporator section creates the main thermal resistance resulting in the largest temperature drop, which consequently affects the performance of the heat pipe. The idea of the heat pipe time constant is introduced in this work to describe the transient characteristics of the flat plate heat pipe and an empirical correlation for the time constant in terms of input heat flux is presented. Correlations for the maximum temperature rise and maximum temperature difference within the heat pipe are also presented. The experimental results at steady state were compared with the analytical results and found to be in good agreement. This work constitutes the first detailed experimental investigation of a flat plate heat pipe.

Transient characterization of flat plate heat pipes during startup and shutdown operations

Analytical models for predicting the transient performance of a flat plate heat pipe for startup and shutdown operations are developed in this work. These models can be utilized separately for a startup or a shutdown operation, respectively. The two models can also be combined together to simulate the thermal performance of a flat plate heat pipe in cyclical startup and shutdown operations. The transient temperature distributions in the heat pipe walls and wicks are presented in this work. The results reveal that the thermal diffusivity, the thickness of the wall and the wick, and the heat input pattern affect the heat pipe time constants. The wicks create the main thermal resistance resulting in the largest temperature drop in the heat pipe, thus substantially influencing the performance of the heat pipe.

Computed Flammability Limits of Opposed-Jet H/O/CO Diffusion Flames at Low Pressure

A narrowband radiation model is coupled to the OPPDIF program to enable the study of one-dimensional hydrogen‐ oxygen diffusion e ame over the entire range of e ammable stretch rates. The e ame characteristics and the extinction limits at a low pressure of 1.013 kPa are sought, with the amount of carbon dioxide dilution level and stretch rate as parameters. The conditions studied are particularly relevant to Mars exploration. In addition, a e ammability map is presented using these two parameters as coordinates. Both the high-stretch blowoff and the low-stretch quenching limits are found. Theexistence of an absolute carbon dioxide dilution limit, above which the diffusion e ame is not possible, is demonstrated. Low-stretch diffusion e ames at low pressures are unusually thick, with e ame temperatures substantially below those of the adiabatic e ames. This large temperature drop results from the combined effect of e ame radiation and limited gas residence time in the e ame, and may be particular to the hydrogen‐ oxygen chemical kinetics. One of the novel features of the application of the narrowband radiation model is the inclusion of Doppler broadening, which is shown to be important in low-pressure e ames.

Thermomechanical aspects of dynamic crack initiation

Dynamic fracture is generally addressed as an isothermal phenomenon. When specific phenomena such as shear band formation and propagation are involved, the analyses include thermomechanical conversion of strain and/or fracture energy into heat (thermoplasticity). In this case, it has been shown that very significant temperature rises can develop which cause softening of the crack-tip material. In these works, thermoelastic temperature changes at the tip of the crack are implicitly neglected. In a recent work, we have questioned this issue and shown that for a stationary crack subjected to transient loading, adiabatic thermoelastic effects were noticeable, thus causing a large temperature drop in the elastic zone surrounding the crack-tip (Rittel, 1998). In the present work, we pursue this line of investigation by presenting additional experimental results about temperature changes ahead of a dynamically loaded crack in commercial polymethylmethacrylate. We investigate mode I and mode II loading configurations. We observe, as expected, that the temperature drops for mode I loading while it rises for the mode II case. In each case, the crack initiates during the phase where the temperature changes (drop or rise). While showing that thermoelastic aspects of fracture should certainly be taken into account, the present results indicate that thermomechanical aspects in general should not be overlooked when addressing dynamic crack initiation.

Entropy analysis of wet-surface cooling system for air flow

SUMMARY A wet-surface heat-exchanger, where the e§uent air is moistened, is analysed according to a thermodynamic theory and data of experimental tests. An entropy generation function, which takes into account the changes of temperature and humidity of air, is derived. The analyses show that it is thermodynamically possible, without any cooling machine, to achieve extremely large temperature drops. With certain parameters the system results in a maximum rate of entropy generation which is used to analyse di⁄erences between two di⁄erent methods of spraying the moistening water. Approximal limits for the eƒcient working fluid mass flow value are also given. ( 1998 John Wiley & Sons, Ltd.

Numerical investigation of convective flows of a highly inhomogeneous gaseous medium

Equations of convection in a gas are analyzed and a computational algorithm that is efficient when the condition μgh/RT≪1 is satisfied for an arbitrary (fairly large) temperature drop is proposed. Calculations of convection occurring in steady nonuniform heating of walls are performed for axisymmetric argoncontaining volumes of different shape. Results of these calculations demonstrate the establishment of steady and periodic motions of the gas.

A comparative study on the thermal characteristics of vertical-cavity surface-emitting lasers

This paper presents a comparative study of the thermal characteristics of four types of vertical-cavity surface-emitting lasers (VCSELs). They include the dielectric-isolated top-emitting p-type substrate VCSEL, the ion-implantation isolated top-emitting n-type substrate VCSEL, the dielectric-isolated bottom-emitting n-type substrate VCSEL, and the junction-isolated etched-well n-type substrate VCSEL. Microscale electrical and heat conduction effects are taken into account by employing reduced and anisotropic electrical and thermal conductivities for the Bragg reflectors. Solutions for both the electrical potential and temperature fields are obtained numerically. The calculated device temperature rise compares well with available experimental data. The simulation results show that among the studied lasers the p-type substrate laser has the best intrinsic thermal performance. Local temperature distributions in these lasers reveal that a large temperature drop occurs across the Bragg reflectors. This study demonstrates that the AlAs concentration and the structure of the Bragg reflectors strongly affect the device temperature rise due to the difference in thermal conductivity. The study also suggests that a thicker top contact should be used to reduce the thermal resistance of these devices.

Numerical modeling of solidification and convection in a viscous pure binary eutectic system

The solidification and convection of the pure binary eutectic silicate system diopside-anorthite (Di-An) is numerically modeled. A mass-weighted enthalpy of fusion is used to account for the second solid phase (An) which crystallizes at the solidus temperature. Variable under-relaxation is used to aid convergence of the momentum equations in our implementation of the SIMPLER algorithm used to solve the two-dimensional continuum conservation equations. Numerical experiments of the solidification of Di80 melt show that a large temperature drop occurs across the solid and mush regions which decreases convective vigor in the liquid. Interesting compositional segregation patterns are produced during the solidification of Di80.

Temperature along the surface of modified fiber tips for Nd:YAG laser angioplasty

For laser angioplasty probes, the thermal properties of the probes will primarily determine their mechanism of action. We examined the absorption, temperature increase, and probe degradation of transparent contact probes (hemispherical contact probe and ball-shaped fibers) and metal laser probes coupled to a continuous-wave Nd-YAG laser. Temperature was recorded by means of thermocouples and the measurements were corrected for direct light absorption by the thermocouple. During 15 W, 1 s exposure, the peak temperature rise of the hemispherical contact probe in contact with tissue dropped from approximately 1,000 degrees C at the front end to below 45 degrees C (95% drop) at the lateral side. In contrast, during continuous exposure the peak temperature rise of metal laser probes in contact with tissue dropped from 560 degrees C at the front end to near 400 degrees C (30% drop) at the 5.5 mm proximal rear end. During exposure in blood or tissue, the transparent contact probes became contaminated. Their absorption increased from 5 to 33% and the probe deteriorated. Repeated use of metal laser probes in blood resulted in a higher temperature at the rear than at the front end due to backburing of the fiber. Owing to the large temperature drop along the surface of transparent contact probes, the area of thermal destruction is limited to the tissue in front of the probe, whereas along the entire surface of metal laser probes the tissue will be affected. The large difference between these temperature distributions should be respected during clinical application of the transparent contact probe and the metal laser probe.

Optimizing the annealing of steel 08Yu sheets in rectangular bell-type furnaces

In the production of steel 08Yu (GOST 9045-80) of category 1-OSV-A at our combine, stacks of cold-rolled metal are annealed in rectangular gas-fired bell-type furnaces. The furnace stand is not heated from below, and there is no forced circulation of the protective atmosphere. The temperature of the metal is checked by thermocouples installed in the bottom (t b) and top (t t) levels of the charge. The temperature is controlled automatically (by a twoposition controller). The signal for switching of the automatic system comes from the readings of the top thermocouple. When the technology just described is used for annealing (Fig. la), there is a large temperature drop (up to 70~ over the height of the stack. As a result, the physicomechanical properties of the metal over the volume of the 65-ton charge are nonuniform, as well as being unsatisfactory low. To solve these problems, we developed a new regime for the recrystallization annealing of steel 08Yu (Fig. 1 b) which includes four resettings of the control temperature tt: 670-720~ -period I (first pulse); 600-650~ -period II; 670-720"C -period I (first pulse); 600-650"C -period II; 670-720"C -period III (second pulse); 680-660~ -period IV. This regime can be realized by means of any modern devices used for programmed control of production processes. Without lengthening the annealing operation, this regime makes it possible to hold the metal at certain temperatures in the bottom part of the stack. This metal is subjected to intermediate (rl) and final (r 2) holds. The holds improve heating and the physico-mechanical properties of the product. Strictly speaking, the term "hold" is applicable only to the top level of the charge. In the middle and bottom parts of the stack, during holds there is some increase in the temperature of the metal. This increase has to do with heat-transfer conditions inside the charge and its thermal inertia. The duration of period I was changed within the range 5-16 h. An analysis showed that a duration within the range 10-13 h ensures the greatest values for elongation (81o), the coefficient of normal plastic anisotropy (R), and grain size and the lowest value of hardness (HRB). The duration of the pulse in this period should be minimal -0.5-6 h. The maximum reduction in HRB for period I and III (Fig. lb) is seen at a control point t t = 700"C. An increase in the control point to 720"C (particularly in period III) is accompanied by an increase in the hardness and the danger of sticking of the sheets. A pulse duration of 0-1 h in period III will produce a minimal value of HRB, while a pulse duration within the range 7-17 h will result in higher values of HRB, IE, and 810 and lower values of cry. The term "pulse duration" in annealing periods I and III means the time t t is held at 700"C. Having optimum holding parameters in the top part of the charge (period II, t t 600"C, duration 6-7 h) somewhat improves the microstructure, hardness, and Erichsen cup depth IE with a I0-12% reduction in fuel consumption. The best characteristics in the bottom part of the charge are seen with 500-5600C for t b and a holding time of 10 h (annealing period II). A reduction in temperature to 450-550"C leads to an increase in strength properties and a reduction in grain size (to number 9). For sheets measuring 0.8 x 800 x 1510 mm, holding time r 1 is limited to 6-10 h. In annealing sheets up to 1.5 mm thick, holding time is cut to 2-4 h. The duration of the final hold (r~), with a temperature of 640-660"C for t b, should be at least 4 h. Lengthening this hold to 15 h reduces hardness. The overall duration of the two-stage pulse heating regime (38-40 h) is the same as in the continuous annealing practice. Thus, the optimum values of the main parameters for the above-developed annealing regime are as follows: pulse temperature 700"C; holding time at this temperature 10-13 h; duration of the second pulse 0-I 7 h; temperature and duration of the intermediate hold in the least-heated parts of the charge 500-560 and 6-10 h. By varying these parameters, it is possible tO change the physico-mechanical properties of the steel in the required direction. The regime for two-stage pulse annealing of cold-rolled steel 08Yu in bell-type gas-fired rectangular furnaces, without forced circulation of the protective atmosphere or bottom heating of the stand, was optimized in accordance with

The Sea-Breeze Front Analytical Model

Abstract Analytical solutions to the nonlinear equations of motion are used to describe the sea breeze front. It is found that the front can develop when the atmosphere stratification in the heated layer is neutral or unstable. The temperature drop due to front passage is proportional to the square of the difference between the front speed and the synoptic wind. This square of the difference, on the other hand, is proportional to [2θ(s)/θm]ga where θ(s) is the mean potential temperature drop across the front and a is the front radius. Fronts which have the same speed of propagation and size, have a larger temperature drop when the synoptic wind blows in the opposite direction to the direction of propagation. Onshore winds associated with strong convergence are obtained in the lower part of the front, while the return current associated with strong divergence is observed in the upper part. The vertical velocity reaches values of some meters per second in the front region. The front propagation is studied...

Transverse thermal dispersion and wall channelling in a packed bed with forced convective flow

An analysis has been performed to study the transverse heat transfer characteristics of a fully-developed forced convective flow in a variable-porosity packed bed bounded between parallel plates, separated at a distance 2 H * and heated asymmetrically. The Brinkman model, with the variable permeability approximated by an exponential function, is used as the momentum equation. The method of matched asymptotic expansions is employed to obtain an analytical solution for the velocity distribution in the packed bed. To account for the higher thermal resistance near the wall of a packed bed, a wall function is proposed for the transverse thermal dispersion process. With the introduction of the wall function, the large temperature drop near the wall of a packed bed observed in forced convection experiments can be reproduced theoretically. In agreement with experimental data, the predicted surface heat flux increases with increasing Peclet number and decreasing d p */ H *.

The Onshore Surge of Marine Air into the Pacific Northwest: A Coastal Region of Complex Terrain

Many coastal locations around the world experience rapid transitions from warm, dry continental air to cool, moist marine air. These onshore “pushes” or surges of marine air can be accompanied by strong winds, large temperature drops and a substantial increase in low clouds. A detailed case study of a typical Pacific Northwest event as well as a composite of several events are presented. It is shown that all major surges are initiated by synoptic-scale changes and that the mesoscale topography of the region “amplifies” the synoptic signal. Annual, monthly and diurnal climatologies are discussed. This paper also discusses the origin of associated West Coast phenomena such as heat troughs and narrow coastal pressure ridges.

Sedimentation in epithermal veins of the Bohemia mining district, Oregon, USA: Interpretations and significance

Open spaces in epithermal veins of the Bohemia mining district, Oregon, USA, filled with sediments during hydrothermal activity. These sediments consist mainly of chalcedony, rock fragments, and vein quartz fragments. In addition, hematite is deposited during stage three of the vein development. Observed sedimentary features include draping laminae, erosion surfaces, slumping, and graded bedding. Such sediments can be used for reconstruction of the original orientation of vein systems, because the sediment laminae are initially deposited horizontally. Vein sediments record variations in fluid flow due to self-sealing, fracturing, and cessation of hydrothermal activity. Investigation of vein sediments therefore provides an additional tool to unravel the geologic history of epithermal systems. The chalcedonic vein sediments record large temperature drops and highly silica supersaturated waters, probably due to fracturing and pressure release. Hematitic vein sediments indicate sulfide deficient hydrothermal fluids.

Numerical study of high Rayleigh number convection in a medium with depth-dependent viscosity

The equations of motion are solved numerically for a Boussinesq fluid with infinite Prandtl number in a square 2-D box where the viscosity increases with depth. Three heating modes are employed: bottom heating, internal heating, and half bottom and half internal heating. In all cases the boundaries are free slip. The range of Rayleigh numbers employed is 10^4-10^7. The viscosity increases as 10^(β(1-y)), where y is distance measured from the bottom upwards and β is a free parameter. In the bottom heated cases, the convective velocities slow near the bottom and result in a large temperature drop between the bottom boundary and interior compared with the top boundary and the interior. This results in increased buoyancy in the ascending limb. In the internally heated case, the flow in the top half of the box resembles Rayleigh-Benard convection and in the bottom half it approaches a conductive thermal regime for β greater than about 2. In this case the top surface heat flux decays from ascending to descending limb and the ascending and descending limbs become more equal in their buoyancy. Increasing β decreases the efficiency of heat transport, but has little effect on the exponents of Nu-Ra and Pe-Ra relations. There is a larger decrease in heat transport efficiency for a given β in the bottom heated case compared to the internally heated case.

Radiative tearing - Magnetic reconnection on a fast thermal-instability time scale

view Abstract Citations (47) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiative tearing - Magnetic reconnection on a fast thermal-instability time scale Steinolfson, R. S. ; van Hoven, G. Abstract Two energy modification mechanisms which are known to occur in sheared magnetic fields are the tearing and thermal instabilities. These processes can be studied separately with formalisms incorporating just the effective driving mechanism of interest (finite resistivity for the tearing mode and unstable radiation for the thermal mode). A model which includes both effects, and a temperature-dependent resistivity, indicates that modified forms of these two instabilities may coexist for identical physical conditions. When they are isolated computationally, one can show that their limiting growth rates are approximately those of the uncoupled instabilities. The spatial structure and energy content of these two new hybrid processes are then individually examined and are found to differ considerably from those obtained from separate treatments of the driving mechanisms. The faster radiative instability, which has a hydromagnetically scaled growth rate like the condensation mode of the thermal instability, is shown to involve a substantial amount of magnetic field reconnection. This can be partially explained by a large temperature drop (or resistivity rise) at the X-point. The island width of the Coulomb-coupled radiative mode is 30 percent of that produced by a comparable level of the slower tearing instability. In addition, the perturbed magnetic energy in the radiative instability is 5 times that of the perturbed thermal energy, indicating an appreciable modification of the initial magnetic structure. Publication: The Astrophysical Journal Pub Date: January 1984 DOI: 10.1086/161623 Bibcode: 1984ApJ...276..391S Keywords: Magnetic Field Reconnection; Magnetohydrodynamic Stability; Solar Flares; Solar Magnetic Field; Tearing Modes (Plasmas); Thermal Instability; Lorentz Force; Resistance Heating; Reynolds Number; Solar Atmosphere; Solar Physics full text sources ADS |

On optimum disposal of waste heat

The irreversibility associated with waste heat disposal across an exchanger represents an inherent trade-off point in the design of power plants. On the one hand, a large degree of irreversibility from a large temperature drop across the heat-transfer surface is detrimental to the thermodynamic efficiency of a cycle that produces waste heat. On the other hand, reduction of irreversibility can be achieved only by employing a larger heat transfer area. Therefore, one expects that there must be an optimum design that maximizes the work output per unit of heat transfer area for a given cycle. The efficiency at this optimum condition should be much closer to efficiencies in comparable cases of “good” design, and it must serve as a much more reasonable yardstick in assesing engineering merit. To exemplify this type of optimization, a Carnot engine cycle with a finite heat transfer coefficient during its heat rejecting process is analyzed in detail. An optimizing temperature drop accompanying the transfer of waste heat is displayed, and the maximum efficiency is calcuated. Influences of the heat transfer at the high-temperature side and the entropy produced during (real) adiabatic processes are also assessed to show the realistic potential of this simple design technique.

Study of the heat transfer under large temperature drops during cooling with a stream of liquid nitrogen mist

Results are shown of feasibility studies concerned with increasing the coefficient of heat transfer to liquid nitrogen under large temperature drops.

Experimental investigation of heat transfer in turbulent flow of air in a circular tube with large temperature drop and cooling of the air

Experimental investigation of heat transfer in turbulent flow of air in a circular tube with large temperature drop and cooling of the air

Thermal stress condition of experimental long blocks during construction period of the bratsk hydroelectric station dam

1. The actual on-site investigations confirmed the correctness of the temperature condition as determined for the long experimental blocks. Maintenance of this condition assures a favorable development of thermal stresses, but deviation from it (large temperature drops, deep freezing, intensive cooling) results in surface cracks at the edges of blocks. 2. The formation of a field of thermal stresses in the monoliths is caused by the simultaneous action of their overall cooling and the non-uniform temperature distribution along the longitudinal sections. The following are considered to be favorable factors: a minimum temperature difference between adjacent blocks, low concrete mix temperature, and a prolonged process of exothermic heating. The compressive stresses, conditioned by this process, to a considerable degree compensate the tensile stresses arising from cooling of blocks and, obviously, prevent cracks from forming in the central zone of the monoliths. It is questionable, however, whether under the Siberian conditions the creation of the surface cracks in the long blocks can be avoided. From this standpoint, they have no advantage as compared to the blocks of columnar section. 3. The stresses in the reinforcement of the plate, separating the long blocks from the columnar ones, reach high magnitudes due to seasonal openings of the inter-columnar joints covered by the plate. This should be taken into account in calculating the reinforcement.