An approximate analytical model of the transient response of a circular foil heat-flux gauge to a step change in incident radiant heat flux is described. This model demonstrates that the sensitivity and the transient response of the gauge are significantly affected by heat flow from the foil to the center lead wire. Results obtained for the transient response to a step change in incident radiant flux from the model agree closely with experimental results from several gauges with different sensitivities. Convolution of the present model and previous exponential models of the unit step response, with a transient incident flux represented by a terminal ramp, shows that the response predicted by a previous exponential model differs significantly from that predicted by the present model.
In this study, an analytical investigation is made of a method of shielding a surface from parallel beams of thermal radiation by distributed injection of a fluid containing absorbing particles into the boundary-layer flow over the surface near the stagnation point. Superposition of various solutions is used to obtain results for a wide range of parameter values from a relatively small number of functions. Three optical conditions of the surface are considered: Specular reflection, diffuse reflection, and black. It is found that injection of absorbing particles helps to shield the surface if the injection rate is relatively high.
A parametric study of the thermal performance during charging of a full-scale radial inlet diffuser in a cylindrical stratified chilled water storage tank was performed by applying factorial experimental theory to the results of simulations performed with a validated computational fluid dynamics (CFD) model. Part 1 describes the development and validation of the CFD model. Part 2 summarizes the results of the parametric study. Dimensional storage tank and inlet diffuser parameters having the potential to influence inlet thermal performance were identified, then formed into dimensionless groups using Buckingham Pi analysis. These included the inlet Richardson number (Ri), inlet Reynolds number (Re i ), ratio of diffuser radius to tank radius (R D /R w ) and ratio of diffuser radius to diffuser inlet height (R D /h i ). Thermal performance was measured in terms of thermocline thickness and equivalent lost tank height. Sixteen simulations comprising a full 2 k factorial experiment were completed and analyzed. Parameter ranges considered were as follows: Ri from 1.0 to 11.1, Re i from 1,000 to 12,000, R D /R w from 0.2 to 0.4 and R D /h i from 5 to 10. Within these ranges, Ri, R D /R w and R D /h i were found to be of first-order significance while Re i was not. Regression models of thermal performance metrics as functions of Ri, R D /R w and R D /h i that are sufficiently simple to be useful for design were developed. These models agreed well with CFD simulations from which they were derived and with field data. INTRODUCTIONNaturally stratified chilled water storage tanks are widely used in large chilled water systems for load shifting. In a naturally stratified tank, cooler, denser water is stored beneath warmer, less dense water. The two bodies of water are prevented from mixing by buoyancy forces. A relatively thin thermal transition layer called a thermocline forms at the interface between warm and cool water.In order to create and maintain good stratification, cool water must always enter and leave a stratified tank at the bottom and warm water must enter and leave at the top. Diffusers at the top and bottom of the tank reduce inlet velocity and promote stratification. The characteristics of the diffuser have a significant effect on thermal performance because of their influence on thermocline formation.The radial parallel plate design is one of two types commonly used in cylindrical tanks. A radial diffuser consists of a plate located near a spreading surface, which is the floor of the tank in the case of a lower diffuser and the water free surface in the case of an upper diffuser ( Figure 52 HVAC&R RESEARCH 1). Inlet flow from a radial parallel plate diffuser enters a cylindrical tank as a horizontal current that flows outward radially from the perimeter of the gap formed by the diffuser disk and spreading surface. During charging, cool water enters the tank through the lower diffuser as warm water is withdrawn through the upper diffuser. During discharging, flow reverses and cool water is withdrawn through...
An investigation has been undertaken to study the thermal stresses in a cylindrical NAP fuel rod. The study has consisted of two phases, one analytical and the other experimental. In the analytical phase, a mathematical model of a fuel rod has been developed in which the rod is considered to be a long solid circular cylinder cooled by emission of thermal radiation. A closed form of solution for the temperature distribution could not be obtained, so numerical results for the temperature and stress distributions were obtained by finite difference methods. The results predict that relatively small stresses are produced when the fuel rod is cooled in a temperature range in which brittle fracture can reasonably be expected. The experimental phase, which is not yet complete, was undertaken to verify the predicted temperature and stress distributions. Rather than using an actual fuel rod in the experimental program, which would present formidable safety and security problems, a modeling technique is being used. The theory of similitude will be employed to relate the results obtained from the physical model to those which one would expect to obtain from a fuel rod. An embedded strain gage technique is being used to measure internal strains.
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