Thermal plumes from staged multiport diffusers in a uniform quiescent environment

Abstract

The behavior of thermal plumes discharged from staged diffusers has been investigated experimentally. A staged diffuser is a type of submerged multiport diffuser characterized by an offshore orientation of the individual nozzles. It is commonly employed for the discharge of heated water from coastal power plants into the ocean because of its effective mixing capability, which does not depend on the prevailing longshore-current directions. Experiments have been performed to measure the temperature distribution of the plume in quiescent receiving water. The three-dimensional thermal field is reconstructed from the results of two groups of measurements, the centerline experiments in which the temperature in the vertical plane along the diffuser axis is measured, and the scanning experiments in which the lateral temperature profiles are measured. The emphasis is on a homogeneous ambient receiving water, but a few illustrative experiments with ambient stratification have been carried out. The number of variables associated with the problem is very large, making it difficult to perform a generic study. Nevertheless, five of the variables (the number of ports, n, the initial jet diameter, D[sub]0, the horizontal orientation of the jet, [Greek alpha], the total discharge flow rate, Q[sub]T0, and the water depth, H) have been examined. Experimental observations support the hypothesis of a near field dominated by momentum jet mixing, and an intermediate field dominated initially by turbulent mixing and eventually by gravitational spreading. H, n and D[sub]0 are the governing parameters in both the near field and the intermediate field. By coupling dimensional analysis with experimental results, several empirical relationships have been established to give a first-order approximation relating the mean characteristics of the plume to the governing parameters. It is found that the near-field dilution can be described adequately by the simple jet model with an adjustment factor based on n. The dilution in the intermediate field, however, s relatively insensitive to n. It is also concluded that the horizontal orientation of the jet +-25[degrees] to the diffuser axis, helps to spread the plume over a wider extent, thereby reducing the maximum temperature rise. Results from stratified experiments indicate that for weak stratification, the dynamics of the plume are not significantly modified.

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