Contact Resistance Demonstration

Contact resistance is the discontinuity of heat transfer between two separate solids.The smoother the two contact surfaces are, the less contact resistance will be and the greater the heat transfer.This is an important part of the heat conduction field.This paper deals with one very inexpensive way to study contact resistance and to teach students about heat transfer across two surfaces.Starting with a 1-inch diameter steel shaft, 21 inches long, 6 holes (1/8-inch diameter) were drilled to the center line.These holes housed K-type thermocouples.One end of the shaft was machined smooth while the other was cut with a torch and cleaned up with a grinder.It was irregular and bumpy which gave inconsistent contact.By using a hot plate (for cooking) along with a metal plate to supply the base heat, the shaft was placed in the vertical position with either the smooth or rough end against the plate.The shaft was held in place by a chemistry flask stand.Once the temperatures of the shaft stabilized, these temperatures and the temperature of the base plate were recorded.The shaft represents a round fin and the temperature dropped off according to the round fin formula.These temperatures were plotted and curve fit with a 4 th degree polynomial function.Using this equation, the fin distance was set to zero to find the fin temperature at the interface.The derivative of this equation at the fin length of zero, gave the slope of the line at the interface.This slope was used to find the heat transfer.With these values, the contact resistance was calculated.The shaft was flipped to the other end and the procedure was repeated.The rough end had a higher contact resistant and a higher base plate temperature.The smooth end, made better contact with the base plate and was more effective in transferring heat to the fin, thus lowering the base plate temperature.