Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Mechanical Engineering


The study of natural convection heat transfer is of significance in several areas of technology. Natural convection represents a limit on the heat transfer rate and this becomes a very important consideration for situations in which other modes are either not possible or not practical. Natural convection from vertical surfaces with some sort of large-scale surface roughness elements is encountered in many technological applications. Of particular interest is the dissipation of heat from electronic circuits, where component performance and reliability are strongly dependent on operating temperature. Natural convection is an inherently reliable cooling process. Circuit boards represent a naturally enhanced free convection situation. In other applications, it may be necessary to enhance the surface to achieve the desired temperature level;The present research was undertaken to provide information on the nature of heat transfer from rough surfaces. An interferometric technique was used to experimentally measure local heat transfer coefficients. This method is essentially non-intrusive and therefore extremely suitable for the study of low-flux phenomena such as natural convection;Several different types of surfaces were studied--namely, repeated ribbed, stepped, and sinusoidal surfaces. The effects of parameters such as protuberance height-to-spacing ratios, amplitude of spacing, conductivity of ribs, leading edge geometry, and angle of inclination were studied;It was found that heat transfer enhancement was possible in laminar natural convection using transverse roughness elements with proper sizing and shape selection. The heat transfer from the ribbed surfaces was found to be less than that from a plane flat plate of equal projected area. It was found that stepped surfaces could be used to improve the heat transfer performance relative to a plane flat plate of equal projected area. The study indicated the presence of an optimum step-pitch-to-height ratio. The performance of sinusoidal surfaces was not significantly different from that of a plane flat plate of equal projected area at low amplitude-to-wavelength ratios. The heat transfer performance decreased as the amplitude-to-wavelength ratio increased. The study also showed that roughness elements induced an early transition to turbulent flow.



Digital Repository @ Iowa State University,

Copyright Owner

Sushil Hiroo Bhavnani



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158 pages