Measuring Heat Transfer Coefficient during Condensation Inside Channels

The heat transfer coefficient during condensation of a pure saturated vapor is not a directly measurable quantity, but it can be indirectly obtained from the measurement of heat flux, saturation temperature, and wall temperature. Different techniques can be found in the literature for the measurement of heat transfer coefficient, and their applicability is mainly determined by the desired accuracy of the measurement and by the shape and diameter of the channel. Basically, the heat flux can be determined in two ways: by using heat flux sensors or from a secondary fluid. Adjectives such as averaged, quasi-local, local, cross-section-averaged can be employed to distinguish among the different types of heat transfer coefficient measurements. In the present chapter, some experimental techniques that use a secondary fluid for heat removal are presented highlighting favorable characteristics, limitations, and possible drawbacks. The experimental approaches based on Wilson plot method and on direct wall temperature measurement are first presented. Then, the reader will be introduced to a local approach (specially developed for small-diameter channels). The problem of estimation of heat transfer coefficient uncertainty will also be addressed with reference to each experimental technique considered here.