Lower Heat Transfer Rate Research Articles

Characteristics of pool boiling heat transfer in the vicinity of the critical heat flux point. (The relations between bubble motion and heat flux fluctuation)

In order to study the mechanism of critical heat flux of pool boiling, measurement of heat flux fluctuation and high speed photographic observation were carried out simultaneously on a constant temperature surface heated by electrically controlled Joule heating, in the vicinity of a critical heat flux point. They revealed the following : (1) when the surface is covered by a coalesced bubble, heat flux begins to increase because evaporation is enhanced ; (2) heat flux begins to decrease either while the surface is covered by a coalesced bubble due to dryout of the liquid on the surface, or just after nucleation begins under the rising bubble due to a relatively low heat transfer rate of nucleation ; and (3) heat applied from the time the surface is covered by a bubble until the time heat flux begins to ducrease does not vary widely and corresponds to latent heat of liquid about 11 -μm thick.

Multiple steady-state solutions for horizontal buoyant flows in cold water

The buoyancy force arising from temperature gradients in cold water is bidirectional if the temperature range spans that of the density extremum. many large scale and subtle consequences of this are now known, for some of the most common flow configurations. the first calculations for horizontal flows encountered a wide range of conditions for which boundary-layer solutions were not obtained. results here narrow that range considerably. they also accurately determine the residual gap of no solutions, in terms of a density extremum parameter r. of equal interest, these calculations have found multiple solutions over ranges of r, on each side of the gap. in some regions, the properties of the second set of solutions are very different and unusual, among buoyancy induced flows. very weak flows are found which, nevertheless, may be very unstable. extremely low heat transfer rates arise under some conditions.

The optimum configuration of natural convection loops

A theoretical study is presented of the influence of loop configuration on natural convection loops (thermosyphons) heated from below and cooled from above. The loops consist of two insulated vertical branches with two horizontal connections, one of which is immersed in a constant temperature heat source and the other in a constant temperature heat sink. A one-dimensional model is utilized to determine the effects of the conduit's cross-sectional area and the aspect ratio for an optimum loop configuration. The coupled momentum and energy equations are solved and the heat transfer rates, mean velocities and temperature distributions are obtained for both laminar and turbulent flows. It is found that the influences of the aspect ratio and cross-sectional area of the loop can be combined into a single dimensionless parameter P 0 = C 3(1 + A) (W r Pr) C4 Gr , where Pr, Gr r , A, r and W are the Prandtl number, the Grashof number based on the tube radius, the aspect ratio, the tube radius and the length of the heat transfer sections respectively, and C 3 and C 4 are constants. The criterion of optimum loop configuration for maximum heat transfer rate is determined in terms of this parameter. The variation in loop configuration results in opposite effects for plus and minus variations from the optimum value of P 0. For loops with P 0 less than the critical value the heat transfer rate can be enhanced by increasing the length of the heated and the cooled sections. If P 0 is greater than the critical value, then the temperature of the working fluid will approach the wall temperature along the heat transfer sections. Further increase in the length of the heated and the cooled sections will result in a lower heat transfer rate.

Asymmetrically corrugated plate heat exchanger plates

Design flexibility for plate heat exchangers is a measure of the ability to adjust the characteristics of individual channels (semi-)independently to suit a particular application. Conventional plate heat exchangers with approximately sinusoidal corrugations have limited design flexibility since the channels for the two heat- exchanging streams are generally geometrically identical. Corrugations that are assymetrical in profile make it possible to construct physically different configurations from a set of identical plates: channels of different cross-sectional free flow area are feasible. Such plates used in various orientations with respect to each other were found to be less efficient than plates with sinusoidal corrugations because a lower heat transfer rate is obtained per unit of pumping power. However, when the flow rates or allowable pressure drops of the two streams exchanging heat differ greatly, such plates may be advantageous.

Efficiency of blast-furnace-stove checkerwork with bricks having spaced projections

Among the shortcomings of the well-known Cowper checkerwork, made of common bricks laid perpendicularly to each other and forming continuous vertical channels of square cross-section, are the relatively small heating surface, low heat-transfer rate, and the impossibility of transverse gas flow, problems which are avoided in checkerwork of brick with spaced projections. A trial batch of such brick was made in accordance with plans prepared by the Lipetsk Polytechnic Institute and approved by the combine and the manufacturing plant. The trial brick was used in a blast furnace at the Novolipetsk combine. It is determined that commercial trials of checkerwork with bricks having spaced projections, and calculations, confirm conclusions regarding efficiency, reached on the basis of laboratory studies.

Emulsification of thermal energy storage materials in an immiscible fluid

The problems of thermal energy storage are of major importance in the development of intermittent energy sources and the efficient usage of conventional energy supplies. Utilization of latent heat materials for thermal energy storage has been plagued by the build-up of solids on cooling surfaces and the resulting low heat transfer rates. A novel system has been investigated in order to alleviate these difficulties. Small droplets of latent heat material were suspended in an immiscible heat transfer fluid to form an emulsion. The generation of stable emulsions is an empirical art, for which the selection of surface-active agents and the method of mixing play the key roles. A total of 42 latent heat storage emulsion samples were prepared using a diphenyl compound as the organic phase. Most of the samples were prepared using a high speed mixing apparatus. Several emulsified blends exhibited favourable prolonged storage and cycling behaviour. Estimates based on apparent viscosity measurements indicated that high rates of heat transfer could be obtained with this system. Assuming turbulent flow conditions and 60 per cent salt loading, a value for the mean film coefficient of heat transfer was calculated to be about 1045 J/m2 s °C. The concept offers potentially large heat exchanger cost reductions, while retaining 60 per cent of the volume savings attainable in latent heat systems.

Measurements of heat transfer to a flat plate in a dissociated high-enthalpy laminar air flow

Heat-transfer rates from a non-equilibrium hypersonic air flow to flat plates at zero and 12° incidence have been measured in a free piston shock tunnel at stagnation enthalpy levels up to 51 MJ kg. Nozzle flow conditions resulted in test section velocities up to 8·1 km 8 and in an experimental regime in which the free stream was chemically frozen and the flat-plate boundary layer was laminar. Estimates of the gas-phase and surface-reaction Damkohler numbers have been made and the heat-transfer results are discussed in this context. At the highest test-section densities non-equilibrium endothermic gas phase reactions involving oxygen atoms in the boundary layer are suggested as a possible mechanism for the observed low heattransfer rates.

Stability characteristics of a single-phase free convection loop

Stability characteristics of a single-phase free convection loop are presented. In the experiments, water was placed inside a toroidal glass loop oriented in a vertical plane. The lower half of the loop was heated and the upper half was cooled. At low heat-transfer rates and also a t high heat-transfer rates the free convection flow was observed to be steady. For the intermediate range, however, the flow was found to be highly oscillatory. Stability predictions are also developed. The comparison between theory and experiment yields favourable agreement.Observations of unstable behaviour have been reported previously for single-phase fluids in the vicinity of the thermodynamic critical point. In these situations it has been assumed that the unusual behaviour of the fluid properties in the near-critical region necessarily constitutes the underlying cause of such instabilities. In contrast t o this view, analyses by Keller (1966) and Welander (1967) indicate that instabilities can occur for ordinary fluids as well. Results of the present study confirm this contention, since instabilities were clearly observed for water at atmospheric pressure and moderate temperatures.

Semiconductor Surface Thermocouples for Determining Heat-Transfer Rates

Possible methods of increasing the sensitivity capabilities for determining heat-transfer rates associated with wind tunnel testing have been investigated. Techniques utilizing surface thermocouples of conventional thermocouple materials do not provide the necessary temperature sensitivity to low heat-transfer rates. This need for increased sensitivity has resulted in development and evaluation of surface thermocouples fabricated from semiconductor materials. Calibration of the semiconductor surface thermocouple has disclosed temperature sensitivity on the order of 35 times that of chromel-constantan thermocouples. This increased sensitivity has established confidence in the potential value of this concept and further investigation and evaluation are presently being conducted.

Investigation of Separation and Associated Heat Transfer and Pressure Distribution on Cone-Cylinder-Flare Configurations at Mach Five

The separated region across the cylinder-flare junction of a conecylinder-flare configuration and its effect on the overall heat-transfer and pressure distributions were investigated at a Mach number of 4.98. Results are presented for pure laminar and transitional separation and for both laminar and turbulent attached flow on a 15 deg half-angle cone-cylinder model with conical flare afterbodies of 10 deg, 17 deg, 24 deg, and. 56 deg half-angles. The free-stream unit Reynolds number ranged from l.6x10(sup 6)to 5.5x(sup 6) ft(sup -1)and the wall temperature from T(sub w/T(sub aw) = 0.18 to 1.0. The extent of a pure laminar separation and a transitional separation was found to decrease with wall cooling, decreasing flare angle, and increasing unit Reynolds number. The heat transfer at reattachment and downstream was dependent on the type of separation - pure laminar or transitional. Low heat-transfer rates were found downstream of a pure laminar separation. For transitional separation, the heat transfer downstream of reattachment was at the high values associated with a thin turbulent boundary layer, with peak heating in the vicinity of reattachment.

A Method for Improving Heat Transfer to a Boiling Fluid

Heat transfer rates from thermally conductive solids to boiling liquids can be improved by coating the solid with an insulator having sufficient insulation value to adjust the temperature difference between the coated solid and the liquid nitrogen to a value where more efficient heat transfer will result. The method is particularly applicable to conditions where the solid is at a much higher temperature than the boiling liquid. Under these conditions stable film boiling occurs. The solid is then blanketed by a stable, highly insulating layer of vapor resuiting in a low heat transfer rate. By coating the solid with an insulator, it is possible to alter the regime oi boiling from stable film boiling to unstable film or nucleate boiling where higher heat transfer rates are possible. (auth)

LXXIII. Thermal stresses in turbine blades

Summary A theory of thermal stress in turbine blades is developed, on the assumption that at each point of the blade planform the stresses are approximately those that would be set up in a free infinite slab of uniform thickness equal to the thickness of the blade at that point. To justify this assumption it is shown that the thermal stresses in a wedge, assuming plane strain and infinite heat transfer coefficient, differ from those based on the approximate theory by terms which tend to zero uniformly with the wedge angle. More general arguments are also given in § 6. Consequences of the theory are that in cooling the maximum stress occurs at all times near the position of maximum thickness, but that in heating the largest stresses are initially near an edge, though as time goes on their position moves alongthe blade towards the position of maximum thickness, and their magnitude increases. Maximum stress is inversely proportional to thermal conductivity for the lower heat transfer rates, but is less sensitive to it at higher rates. Application to a specific material and comparison with experiment are given in § 7.