APPLICATIONS

Three sets of experiments involving (1) buoyant flow around a protruding heater [77], (2) transient convection in a square cavity a differentially heated fluid layer are discussed in the present section. All of them employ interferometry, to thermal convection.

Buoyancy- Driven Flow around a Protruding Heater    

Buoyancy-driven flow in the vicinity of a protruding heated block copper block placed on a vertical wall and exposed to the ambient is experimentally studies here. The copper block is of height , protrusion , and a length , which is much larger than and . it is located on a vertical Bakelite board with its longest dimension lying in the horizontal plane. All measurements have been carried out at steady state. The average Nusselt number as a function of Rayleigh number has been reported in this work. The thermal wake above the block as visualized by the Mach-Zehnder interferometer is also presented in this study.

The problem addressed here arises frequently in the thermal design of high-performance electronic components such as integrated chips in computers. An overall review of the subject for practical cooling configurations of electronic circuit boards has been presented by Incropera [103]. Heat transfer from arrays of protruding three-dimensional heaters under forced flow conditions have been experimentally studies by Garimella and Eibeck [104]. Experiments on two-and three- dimensional natural convection heat transfer form vertical, discrete, and arrays of flush and mildly protruding heaters have been presented in the literature [105]. Nusselt number and wake size in natural convection for vertical and horizontal protruding thermal sources using thermocouple data have been studies by Kang and Jaluria [106].

The dimensionless parameters of the problem are the aspect ratio Rayleigh number (Ra) and the average Nusselt number (Nu) based on the height of the copper block. Fluid properties are evaluated at the average of the heater and room temperatures. There is some uncertainly in the form of the boundary condition at the heater surface since it can be prescribed as constant temperature or as constant heat flux. The use of a copper block would suggest the former, but since the heater size is quite small in the present study we continue to examine the constant heat flux boundary condition. The respective values of the temperature difference in the definitions if Ra and Nu are computed as and respectively. In all the data presented in this work, the aspect ratio A is equal to two.

Two different heaters of sizes mm and mm have been employed in the present work. Surface temperatures employed are vary from are in the range of . The copper block is electrically powered by a nichrome-wound heater placed behind it. The electrical resistance of the nichrome wire used is 95 ohms/m. the voltage applied to the heater is stabilized using a series of variances. The entire heater assembly is mounted on a bakelite sheet (Fig.29). This sheet is placed vertically in an enclosed test cell that straightens the flow approaching the copper block.

Figure 4.35: Schematic of the experimental apparatus of a protruding heater