| The effect of changing the droplet diameter and the initial speed-ratio (i.e., the ratio of the entering water and air velocities) on the performance of a counter flow direct-contact spray cooler is numerically investigated. The spray cooler is modeled as a spray dispersing water droplets in a full cone into a rectangular insulated duct. The two-dimensional governing equations are solved numerically using a commercial computational fluid dynamics software package. Physical experiments are conducted to validate the numerical model. Inputs to the model include air-side velocity and temperature at inlet, water-side mass flow rate and temperature at inlet, surface area median droplet diameter, and spray cone angle. The surface area median droplet diameter and spray cone angle are measured from still digital images of a spray using image analysis software. Two wall boundary conditions are considered that bracket the range of possibilities for all wall conditions.; The results of the numerical simulations showed that the gravitational force causes the profiles to be skewed towards the bottom, resulting in a non-uniform temperature distribution along the vertical direction. However, this non-uniformity decreases as the initial speed-ratio decreases. Generally, the effectiveness tends to increase as the spray cooler initial speed ratio increases until the maximum effectiveness is reached at an optimum speed ratio. The trend is then reversed beyond this point. The results also show that the effectiveness for depends on the geometry and the hydrodynamics of the air and water droplets, expressed as the speed ratio. Moreover, increasing the droplet diameter decreases the effectiveness. Finally, analytical and semi-analytical expressions are developed to express the dependence of the effectiveness of the spray cooler on its geometry, hydrodynamics of the air and water droplets and droplet diameter. The model is formulated in a non dimensional form to ensure versatility and broaden its application. |
