1. T HE C OOLING E FFECTS OF V ARYING W ATER D ROPLET V OLUME AND S URFACE C ONTACT A NGLE W ITH A M ETAL S URFACE I N A S TEADY S TATE, H IGH T EMPERATURE A IR F LOW Project Presentation MANE 6970 Matthew Noll April 23, 2015

2. INTRODUCTION This project analyzes the effects of changing physical parameters of a water droplet during evaporative cooling of a flat metal plate in a high temperature air flow. Figure from Reference 1

3. BACKGROUND Cooling a high temperature air flow, such as the exhaust flow from a stationary natural gas turbine generator, normally requires spray cooling with droplet sizes on the scale of atomized particles. System limitations may limit the ability to produce an atomized droplet spray. Water quality available for cooling spray Limits in cooling system pressure Spray nozzle availability Additives that affect droplet surface tension Evaporative cooling provides the potential to maintain materials and components that are located in high temperature air flows at much lower temperatures without the need for an atomized spray.

4. METHOD

5. THEORY This paper begins with a 2 dimensional droplet modeled as a half circle on top of a rectangular metal plate in a region of hot air flow. The original droplet diameter is 1mm, and since the droplet is modeled as a half circle, the surface contact angle is 90°. The effects of changing the droplet volume, surface contact angle and separation between droplets are studied, but this paper does not research what methods are used to change these properties of the droplet. In an effort to simplify the study, the results model the evaporation process as a snapshot in time, and the boundary of the droplet is not moving.

6. ASSUMPTIONS Heat and mass transfer of the system are at steady state. The properties of the water are assumed to be constant. The upper “wall” (boundary) of the gas region modeled is assumed to be a slip conditions so that it will not affect the velocity field over the droplet and bottom wall. Pure water is used for the spraying fluid. It is assumed that atomization of the water is not possible with the components of the system, so droplet diameter is assumed to be 1mm and greater. T gas = 280°C T water = 40°C U gas = 0.44 m/s (scaled from mass ratio of gas and water spray) The time step is very small, so the analysis is assumed to be a snapshot in time.

7. PRELIMINARY RESULTS Originally, the wall of the flat plate was modeled as a non-slip condition, which disrupted the velocity field of the hot air flow. The second figure models the droplet in a higher velocity flow and a domain with a smaller height.

8. TEMPERATURE DISTRIBUTION RESULTS The same model was used to find the surface plot of the temperature in the system. However, since the bottom wall of the plate was assumed to be at a constant temperature, the heat flux ended up going from the droplet to the plate rater than from the plate to the droplet.

9. NEXT STEPS Modeling the bottom of the plate as a constant temperature was the issue. By simplifying the model without a metal surface (only diffusion and convection between exhaust gas and water droplet) the correct heat flux from the droplet can be found, therefore finding the cooling capability of the droplet. Droplet Exhaust Gas Droplet Surface Temperature = T sat

10. VELOCITY AND CONCENTRATION FIELDS The figure on the left shoes the wake created in the gas flow by a single droplet, and the right figure shows that the concentration of water in the gas flow increases close to the droplet surface and in the velocity wake.

11. AVERAGE HEAT FLUX ON DROPLET SURFACES

12. SEPARATION OF DROPLETS The average heat flux effect of varying the distance between two droplets on a surface was also studied. The figure below shows a case of temperature variation with a droplet separation of 0.05 m

13. AVERAGE HEAT FLUX ON DROPLET SURFACES

14. REFERENCES 1.Fundamentals of Heat and Mass Transfer, Wiley 2011, Hoboken NJ, 07030-5774 2.S.Semenov, V.M. Starov, R.G. Rubio, M.G. Velarde. Instantaneous distribution of fluxes in the course of evaporation of sessile liquid droplets: computer simulations, Loughborough University Institutional Repository, 2010 3.Capstone C1000 Megawatt Power Package – High-pressure Natural Gas, http://www.capstoneturbine.com/_docs/datasheets/C1000%20HPNG_331044F_l owres.pdf