Experimental studies and mathematical modeling of chilled-water cooling coils

David Raymond Mirth, Purdue University

Abstract

An experimental and analytical investigation of the performance characteristics of chilled-water cooling coils has been performed, and a model capable of predicting the performance of such coils has been developed. The model includes appropriate formulas for calculating the heat transfer coefficients on the air- and water-sides of the coil, as well as a procedure for calculating the condensation rate on the air-side. An investigation of water-side heat transfer revealed that the use of the Gnielinski correlation to predict the water-side Nusselt numbers resulted in more accurate predictions of coil heat transfer rates than did the use of the more commonly chosen Dittus-Boelter correlation. Experimentally determined water-side friction factors were found to agree within 20% of those predicted by equations from the literature. On the air-side of the coil, a new method of modeling the heat and mass transfer has been described. This new model was compared with adapted versions of models previously described in ARI Standard 410-87 (1987), and in a paper by McQuiston (1975). It was determined that the assumptions required by McQuiston's model resulted in an overprediction of coil heat transfer rates. A sensitivity analysis revealed that the experimentally obtained dry- and wet-surface Nusselt numbers were very sensitive to uncertainties in the measured temperatures and heat transfer rates. It was also determined that the use of dry-surface Nusselt number correlations in a coil model resulted in wet-surface heat transfer predictions which were generally within 5% of the experimentally determined value. General correlations for predicting the air-side Nusselt number and friction factor have been developed from the data for the five coils. It was found that, for the wavy fins used in these coils, the Nusselt number was independent of the fin wave geometry. The correlations were inserted into a general cooling coil model, which was then tested by comparing its predicted heat transfer rates and pressure drops with experimental data from the literature. The model was shown to be able to predict heat transfer rates to within about 5% for seven corrugated-fin coils representing a wide range of geometries. The model was able to predict the air-side pressure drop within 25% for three coils for which the fin wavelength and wave height parameters were similar to those of the coils used to develop the friction factor correlation. A parametric study conducted using this model revealed that maintaining a high water velocity is critical to maximizing the heat transfer rate of a chilled-water cooling coil operating at low water temperatures.

Degree

Ph.D.

Advisors

Ramadhyani, Purdue University.

Subject Area

Mechanical engineering

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