Principle

The lumped-mass assumption (negligible internal thermal resistance) is used to infer forced-, natural-, or mixed-convection heat-transfer coefficients on spheres, cylinders, and other shapes.

Objective

This experiment allows the estimation of heat-transfer coefficients that result from external flows. Use of the lumped-mass approximation is a key element to the work, and this experiment can be used to explore the limits of this important experimental convective technique. Also of value is the estimation of the radiation contribution compared to the convective contribution in the total heat loss from a heated object. The basic approach can be used for a variety of geometries in forced-, free-, or mixed-convection arrangements. The description here focuses on forced-convection applications, but the basic apparatus and concept can be used for the other situations as well.

Apparatus

• Constant-speed centrifugal fan with uniform flow outlet and damper on inlet (for forced-flow experiments)

• Hand-held anemometer, propeller type (for forced-flow experiments)

• Thermocouple reference junction and signal readout device (many data loggers combine these functions and can be used). A highly desirable alternative is a computer-based data acquisition system.

• Clock (if time is not recorded with data logger or computer)

• Support stand

• Bunsen burner and lighter

• Two thermocouples, one in ambient air and one with adaptor for test element

• Barometer

• Copper test elements (e.g., sphere, cylinder)

A diagram of the test apparatus suitable for forced-flow experiments is shown in Fig. 12.1.