Search

Forced convection heat transfer from a cylindrical heating element with crossflow occurs in many situations of practical interest, but heat-transfer coefficients for liquids have been reported in the literature only for Reynolds numbers below 200. This paper correlates new data taken with water and ethylene glycol at Reynolds numbers from 40 to 100,000 and Prandtl numbers from 1 to 300. The effects of temperature differences large enough to produce significant changes in viscosity across the boundary layer have also been investigated and are correlated in terms of a viscosity ratio.

Heat transfer data are reported for partially ionized argon flowing in a water-cooled circular tube, 1/2 in. in dia. Experiments were run with initial mixed-mean temperatures up to 21,000 deg R at Reynolds numbers from 140–527 based on equilibrium properties evaluated at the entrance mixed-mean temperature. Measured plasma flow Nusselt numbers computed on an enthalpy basis correlate well with low temperature, constant property results after the first 5–10 diameters of the tube entrance region; closer to the entrance, Nusselt numbers were about 30 percent higher than constant property entrance region predictions. The tendency of the data to approach the constant property solution rapidly was predicted by a laminar finite-difference analysis for plasma flow published earlier [1]. The analysis was modified for the present study to improve its accuracy. The finite-difference theory under predicts the heat transfer in the first few tube diameters; two possible reasons for this discrepancy are the poor resolution in the inlet enthalpy profiles near the tube wall and nonequilibrium between electrons and heavy particles in this region.

The results of an experimental investigation of the frictional and heat transfer effects for gases passing through heated channels with Reynolds numbers from 200 to 2000 and with values of the ratio of the temperature of the wall to the bulk temperature of the gas up to 2.2 are presented. Local friction factors, not previously available, were found to increase much more rapidly with temperature ratio than predicted by existing analysis [1]. Local heat transfer results are compared to and found to agree with the results of Kays and Nicoll [2]. Previous analyses which include the variation of gas properties with temperature are inadequate to explain the substantial increase in friction factor with temperature ratio. The effects of a postulated radial velocity component are examined and are found to be adequate to explain the friction factor and Nusselt number results.

The results of an experimental investigation of the frictional and heat transfer effects in air, nitrogen, and helium in steady, turbulent flow in round tubes are presented. Wall-to-bulk temperature ratios extend from near unity to 2.5, entering Reynolds numbers from 15,000 to 233,000. Dependence of the thermodynamic and transport properties on temperature is shown to be significant in both heat transfer and wall friction and is reflected in the empirical correlations for local and average coefficients which are developed. The results of other investigators who have analyzed or measured variable properties effects are recalculated where necessary to facilitate comparisons.

Empirical data are presented which show the effects of diameter, water velocity, and subcooling on the critical heat flux from an electrically heated, cylindrical lube or wire. The maximum flux which can be accommodated in subcooled nucleate boiling is found to vary directly with the water velocity and subcooling and inversely with a fractional power of the heater diameter. The exponent which describes the diameter dependence is itself a function of both velocity and subcooling. Measurements of the critical flux are reported for water at atmospheric pressure over a range of subcooling from 3 to 100 deg F, velocity from 0.5 to 11 ft/sec, and heater diameter from 0.010 to 0.189 in. Visual and photographic observations indicate a marked effect of subcooling on the flow mechanism near the critical heat flux. High subcooling prevents the formation of the vapor cavity which was described in the previous paper [1] for nearly saturated water, although the failure of nucleate boiling still occurs at the rear of the cylinder and is accompanied by a concentration of vapor in that region.

Visual and photographic observations are used to construct a physical model of the mechanism of transition from nucleate to film boiling on a cylindrical heater. In this paper, interest is focused on forced-convection boiling of a liquid which is near its saturation temperature, while a companion paper deals with the effects of various degrees of liquid subcooling on the peak flux. An approximate analysis is presented of the saturated nucleate-boiling model which predicts the critical flux, and comparisons are made with experimental observations. Measurements of the peak nucleate-boiling heat flux are reported for water at atmospheric pressure over a velocity range of 1.2 to 9.5 feet per second. Resistively heated, stainless-steel wires and tubes of 0.010 to 0.189-inch diameter, the latter with wall thicknesses of 0.006 to 0.028 in., were used. Within these ranges of variables, the critical flux is found to increase with the square root of the velocity and to be independent of heater wall thickness. Only a weak dependence on the heater diameter is observable, but the tendency is for the peak flux to diminish for larger tubes.

An approximate analytical solution is presented for the variation of the local heat-transfer coefficient over the forward half of a uniformly heated sphere. Experimental measurements with water over a Reynolds number range of 5000 to 480,000 and a Prandtl number range of 2.2 to 6.8 give local coefficients which are in good agreement with analytical results. Average heat-transfer coefficients for the uniformly heated sphere are slightly higher than similar results reported earlier [1] for an isothermal sphere. The effect of variations of heat flux on the average heat-transfer coefficient is correlated in a manner similar to that which was used for the isothermal data. Three different duct sizes were used in the experiment to determine the effect of this variable, and the correlations which are presented are based on duct-to-sphere diameter ratios of 2, 2.67, and 4.

Heat transfer from a spherical heating element by forced convection occurs in many situations of practical importance. While the present investigation stems from interest in spherical fuel elements for liquid-cooled nuclear reactors, similar heat-transfer conditions may prevail in chemical process reactors and in other engineering applications. Previously reported measurements of convective heat-transfer coefficients from spheres to liquids have been limited to low Reynolds numbers and to negligibly small temperature differences. This paper correlates these earlier data with new measurements taken at much higher Reynolds numbers and with substantial temperature difference between the heating surface and the liquid. The significance of the larger temperature difference is twofold: Since the viscosity variation of liquids with temperature is usually strong, there may be an important viscosity change across the boundary layer. Furthermore, natural convection effects may not be ignored in regions of large temperature difference and low or moderate Reynolds number. Both of these effects are discussed in the paper.

Aspects of local boiling in forced circulation when the boiling fluid contains small percentages of alcohol are discussed. Particular emphasis is placed on the improvement in smoothness of boiling and in the decrease of average bubble size when the additive is present, as contrasted with boiling distilled water. Experimental results, including photographs, are presented for surface boiling from the outside of a cylindrical stainless-steel tube placed normal to the flow. Data are also presented for the subcooled boiling of distilled water, and comparisons are made with correlations from the literature.