Experimental and theoretical investigation of spray cooling heat transfer on macro and micro level Mariya Nacheva*, Teodor Todorov, Dimitar Dontchev, Juergen Schmidt Institute of Fluid Dynamics and Thermodynamics Otto-von-Guericke-University, 39106 Magdeburg, Germany * DFG-Graduiertenkolleg “Micro-Macro Interactions in Structured Media and Particle Systems” Many technical processes in the metallurgy, process engineering and minimum cooling technology demand a rapid cooling of hot surfaces. Such a cooling can be obtained by different liquids and techniques. The spray cooling is often preferred because of the good adaptability to different cooling requirements. The realization of defined cooling conditions requires the knowledge of the heat transfer coefficient (HTC) on macro level. The cooling fluid, its mass flux and the spray characteristics (velocity and size of the droplets) influence the HTC in the spray cooling. In most research works, the HTC is correlated as a function of the mass flux. However, other studies consider as well its dependence on the droplet size, droplet velocity, material properties and the temperature of the wall. Significant discrepancies can be observed while comparing results obtained by different authors. These differ- ences can be explained by the employment of different test conditions and different nozzles. In the majority of the scientific works, the parameters that influence the HTC were not varied independent- ly from one other. In the present work, the influence of the spray characteristics on the HTC is in- vestigated for stable film boiling above the Leidenfrost point. In this regard, each parameter (mass flux, droplet velocity and droplet size) is varied while keeping the others constant. Previous results were already presented5. Two experimental methods (quasi-stationary and transient) are used for the estimation of the HTC. The quasi-stationary method can be applied only when the fluid impingement density is uniform. If this condition is not fulfilled the vapor film collapses rapidly. The employed experimental set-up is sketched in Fig. 1. A Phase-Doppler-Anemometer (PDA) was used to determine the spray character- istics-the volume mean diameter and the mean velocity of the droplets. Distilled water was used as the cooling liquid. The mass flux was measured by patternator. An electrically heated metal sheet (60 x 43 x 0.3 mm) was used for the heat transfer experiments. The constant electrical current supply (up to 400 A) provides a nearly constant heat flux. The HTC was calculated from the temperature field of the “dry” side of the surface when the calibration of the heat losses on the same side is taken into account. The temperature field of the metal plate was determined by means of the IR- Thermography. The test section was coated, Fig. 2, using a high temperature resistant laquer, whose emissivity was determined as a function of the temperature. An inverse heat conduction algorithm based on the Beck’s function specification procedure1 was used to analyze the experimental data. The HTC can be calculated at the „spray” side since the boundary conditions at all other sides are known and the temperature history at the “coated” side, Fig. 2, can be obtained by means of the IR-Thermography. The computation of the 2D-temperature field, where the surface coating is taken into account, is based on the Finite–Difference–Method (FDM). The inverse method is applicable to both steady and unsteady measurements. The calcula- tions in the case of a nearly uniform impingement density can be simplified. Full cone (Bete CW 25, Lechler 460.406) and flat spray (Lechler 432.404, Lechler 432.366) water nozzles were used. Large variations in the impingement density, droplet diameter and mean droplet Neumann boundary condition spray side IR- camera side PDA Unit PDA Unit Water surface coating Nozzle sheet steele + Newton boundary Transmitter Current Metal condition -- Sheet Receiver unknown IR temperatu IR Unit Unit IR re Camera Pump r couple condition Neumann boundary y Water condition Tank Figure 1 Experimental set-up Figure 2 Scheme of the test section and conditions for the mathematical model velocity are necessary for a systematical studies. Each spray parameter has to be varied nearly independently nt [W/m K] 800 2 700 while keeping the other two approximately fixed. Pres- 600 sure (0.1-0.9 MPa), water flow rate (28-105 l/h), nozzle Heat transfer coefficie 500 400 distance (0.04-0.5 m) and temperature of the test sec- 300 tion (200-600 °C) were the varied experimental pa- 200 ] m rameters. [m 100 30 h dt 0 25 wi 20 t ee 25 20 sh 30 P ar t 35 Typical results are presented in Fig.3 for the nozzle e of th 40 th e sh 45 15 eetle of ngth Bete CW 25, which produces nearly uniform water dis- rt [mm Pa ] tribution. At the rims of the area the vapor film begins to collapse. For the data reduction a small area (25 x 15 Figure 3 Local distribution of HTC over mm) of the centre of the plate was selected. The HTC the surface was calculated for each point of the chosen area. 350 450 Nozzles: w = 450 °C 300 432.404, d_30 = 107 µm; v = 3.7 m/s m = 8 kg/(m²min) 400 Heat tramsfer coefficient [W/(m²K)] Heat transfer coefficient [W/m 2K] 460.406, d_30 = 46 µm; v = 3.7 m/s 350 250 432.366, d_30 = 71 µm; v = 3.7 m/s 300 200 250 v = 3,5 m/s 150 v = 5,5 m/s 200 v = 8 m/s 100 150 v=4,3-4,8 m/s 50 v=6,8-7,2 m/s 100 v=7,5-8 m/s 0 50 200 250 300 350 400 450 500 550 600 650 0 5 10 15 20 25 30 35 40 45 Wall temperature [°C] Water impingement density [kg/m2min] Figure 4 Influence of the droplet size Figure 5 HTC vs. impingement density The behavior of the HTC with the wall temperature, Fig. 4, shows that the influence of the droplet diameter is negligible. Two sets of data illustrating the effects of the droplet velocity, Fig. 5, were obtained by using both experimental methods. The full points in Fig. 5 correspond to the unsteady state method. The droplet velocity is varied from 3.5 to 8 m/s. It can be seen that the impingement density has the strongest effect on the HTC in comparison to other parameters. The HTC increases also clearly with increase in the droplet velocity. Similar results for dependence of the HTC on the velocity ware presented for unstable film boiling by Chen3. 24 Simulation Wruck/Renz 20 Bolle/Moureau Puschmann Evaporation efficiency [%] 16 12 8 4 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 Droplet radius [mm] Figure 6 Micro-Macro-Interaction by spray Figure 7 Model comparison for the evaporation effi- cooling ciency of a single droplet For a better theoretical explanation of the influence of the droplet size and velocity observed our aim is to model the spray cooling on micro level. The direct interaction of single droplets with the wall is considered firstly for dilute sprays. One single drop and a characteristic part of the surface, which dependences on the spray parameters, constitute a representative volume element (RVE), Fig. 6. As the droplet reaches the surface, most of the heat is transferred in the first stage, when the liquid is in direct contact with the material and before a vapor film is formed. After the impact, the drop spreads out to its maximum diameter. Time-dependent droplet propagation is considered and a first approach is presented. From this function and the numerical solution of the transient 2D-heat conduction prob- lem using the enthalpy method and FDM, the heat delivered by the wall can be calculated. This heat is characteristic for a certain drop type and permits the calculation of the HTC by considering of the droplet flux. Results are compared with those of Bolle/Moureau2, Puschmann4, and Wruck6, Fig. 7, Q drop based on the evaporation efficiency v of a single droplet. M drop h v Acknowledgment The financial support of the German Research Foundation (DFG-Graduate College 828 “MicroMacro Interactions in Structured Media and Particle Systems”) is gratefully acknowledged. References 1. Beck, J., Blackwell B., Inverse heat conduction, Wiley-Interscience Publ., 1985. 2. Bolle, L. and Moureau, J. C., Spray cooling of surfaces, in: Multiphase science and technolo- gy, Vol. 1, pp 1-97, McCraw-Hill, New York, 1982. 3. Chen, R.H., Chow, L.C., Navedo, J.E., Effects on Critical Heat Flux in subcooled Water Spray Cooling, Int. J. Heat Mass Transfer, 45 (2002), 4033- 4043. 4. Puschmann, F., Experimentelle Untersuchung der Sprühkühlung zur Qualitätsverbesserung durch definierte Einstellung des Wärmeübergangs. PhD Thesis, Otto-von-Guericke- Universität, Magdeburg, 2003. 5. Schmidt J. and Boye H., Influence of Velocity and Size of Droplets on the Heat Transfer in Spray Cooling, Chem. Eng. Technol., Vol. 24, pp 255- 260, 2001. 6. Wruck, N., Transientes Sieden von Tropfen beim Wandaufprall, PhD Thesis, Shaker Verlag, Aachen, 1999.
Pages to are hidden for
"Micro model for the single droplet and experiments for the spray "Please download to view full document