1. T HE E VAPORATIVE C OOLING E FFECTS OF V ARYING W ATER D ROPLET C HARACTERISTICS ON 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 atomized spray cooling. 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 have materials and components in high temperature flows be maintained 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 10mm, 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. Also, the convective heat transfer coefficient (h) is assumed constant for all models.

6. ASSUMPTIONS

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 shows 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

13. 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

14. AVERAGE HEAT FLUX ON DROPLET SURFACES The average heat flux of the upstream (blue) droplet is affected very little by the presence of the downstream droplet. However, as the separation increases between the droplets, the average heat flux from the downstream droplet increases.

15. AVERAGE HEAT FLUX COMPARISON

16. FUTURE EFFORTS If possible, a future project could focus on connecting the conductive heat flux from the metal plate at a specific temperature into this model. Another step to take would be to model this system with a constantly moving boundary, as an actual evaporating droplet would have.

17. 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