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Effective heat exchange surfaces for vapor generation
Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics 157
Leidenfrost Temperature on an Extremely Smooth Surface Niroh Nagai Shigefumi Nishio
Institute of Industrial Science University of Tokyo Roppongi, Minato-ku, Tokyo, Japan Most of the existing heat-transfer models on transition boiling employ the following hypothesis on the mechanism limiting the so-called liquid-solid contact fraction: activation of preexisting nuclei upon liquid-solid contact is a necessary condition for the liquid-solid contact to be intermittent and localized, and the conventional activation criteria are available to predict the activation of preexisting nuclei in the wetted area upon contact. Supposing this hypothesis to be valid, behavior of liquid-solid contact on an extremely smooth surface such as a single crystal surface is expected to be much different from that on a metal surface of usual roughness. In this report, to examine the hypothesis, the Leidenfrost temperature was measured on single crystal and metal plates. The maximum surface roughness of the former is 0.03 /xm and that of the latter is 1.25/zm. Results of the experiment show that the Leidenfrost temperatures on these two surfaces are not different from each other as long as the surfaces are the same in wettability and the thermal conductivity (or thermal diffusivity). Visual observation of the transient and localized wetted area on the single crystal surface shows that numerous tiny dry spots are formed in the wetted area even at initial surface superheats lower than the minimum superheat to activate the preexisting nucleus of 0.03/zm in radius. These results imply that the hypothesis employed in most of the existing heat-transfer models on transition boiling must be reexamined.
Influence of a Lower Heated Tube on Heat Transfer from an Upper Heated Tube in CFC-114 S. B. Memory
Department of Mechanical Engineering University of Miami Coral Gables, Florida L. R. Lake P. J. Marto
Department of Mechanical Engineering Naval Postgraduate School Monterey, California This paper reports heat transfer measurements from the upper tube of an array of two horizontal, vertically aligned tubes immersed in a pool of CFC-114 at 1 atmosphere. Smooth and porous coated tubes were tested in separate arrays. The upper tube wall was instrumented with eight thermocouples in order to obtain an accurate average wall temperature. For the majority of tests, the heat flux on the lower tube was fixed while the heat flux on the upper tube was first increased up to 100 k W / m 2 and then decreased in order to obtain the familiar hysteresis loop. Tests were also conducted by varying the heat flux of both tubes by the same amount. Tube pitch to tube diameter ( P / D ) ratios of 1.5, 1.8 and 2 were tested. For both arrays, the influence of a lower tube on an upper tube in the natural convection region was small (up to 40%). For a lower boiling smooth tube, a factor of 6 increase in the heat transfer from an upper non-boiling
smooth tube was obtained at heat fluxes below 20 k W / m 2 and all evidence of boiling hysteresis was eliminated. This is attributed to impinging bubbles from below. At higher heat fluxes, with both smooth tubes boiling, the influence of bubbles from below was negligible. The only effect of a lower boiling porous coated tube was to promote slightly early partial nucleation. No other effect of the lower porous coated tube was found at any heat flux due to the high density of active nucleation sites present. However, boiling hysteresis was never completely eliminated. The influence of tube spacing was found to be small.
Effective Heat Exchange Surfaces for Vapor Generation G. A. Dreitser
Department of Theoretical Thermotechniks Moscow Aviation Institute Moscow, Russia I. Z. Kopp
Sankt-Petersburg Technic University Sankt-Petersburg, Russia This paper considers the experimental study of heat transfer for vapor generation on various liquids of different macro and microstructure surfaces. Our use of universal models for the main stages of the vapor generation mechanism (nucleation sites, bubble rise, and departure) enabled us to demonstrate the effect of the surface structure and properties on heat transfer. Good results were obtained to assist in determining the most advantageous conditions of using the effective heat exchange surfaces with assigned properties and structure: cleaning degree, special machining, presence of coatings, finning, etc. A general form of the solutions makes grounds for choosing optimal data on economizer and evaporation sections of heat exchange surfaces of vapor generators for liquid nucleate, transient or film boiling conditions.
Measurements of the Heat Transfer Coefficient and Pressure Drop in Flow Boiling of HCFC and HFC Refrigerants in Smooth and Microfinned Tubes G. Galli L. Laurenti F. Marcotullio A. Ponticiello
Dipartimento di Energetica Universith degli Studi di L'Aquila Localith Monteluco Roio Poggio (Aq), Italy The heat-transfer coefficient and pressure drop in forced flow boiling in a smooth tube and in two types of microfinned evaporators were measured for R12 and R134a. The test section was a straight, horizontal tube, heated by water circulated in a surrounding annulus. The evaporation temperature ranged from 5°C-15°C; the inlet and outlet qualities were 5 and 90%, respectively; and the specific mass flow rate ranged from 180-300 kh/m2s. The heat-transfer enhancement varies from 15%-40% in one of the microfinned tubes, and from 50%-70% in the other, when compared to the performance in a smooth tube in equivalent conditions. The increment in pressure drop for the first tube is not higher than 20% by comparison to the smooth tube, while the second microfinned tube shows an increment of about 100%.
Leidenfrost Temperature on an Extremely Smooth Surface Niroh Nagai Shigefumi Nishio
Institute of Industrial Science University of Tokyo Roppongi, Minato-ku, Tokyo, Japan Most of the existing heat-transfer models on transition boiling employ the following hypothesis on the mechanism limiting the so-called liquid-solid contact fraction: activation of preexisting nuclei upon liquid-solid contact is a necessary condition for the liquid-solid contact to be intermittent and localized, and the conventional activation criteria are available to predict the activation of preexisting nuclei in the wetted area upon contact. Supposing this hypothesis to be valid, behavior of liquid-solid contact on an extremely smooth surface such as a single crystal surface is expected to be much different from that on a metal surface of usual roughness. In this report, to examine the hypothesis, the Leidenfrost temperature was measured on single crystal and metal plates. The maximum surface roughness of the former is 0.03 /xm and that of the latter is 1.25/zm. Results of the experiment show that the Leidenfrost temperatures on these two surfaces are not different from each other as long as the surfaces are the same in wettability and the thermal conductivity (or thermal diffusivity). Visual observation of the transient and localized wetted area on the single crystal surface shows that numerous tiny dry spots are formed in the wetted area even at initial surface superheats lower than the minimum superheat to activate the preexisting nucleus of 0.03/zm in radius. These results imply that the hypothesis employed in most of the existing heat-transfer models on transition boiling must be reexamined.
Influence of a Lower Heated Tube on Heat Transfer from an Upper Heated Tube in CFC-114 S. B. Memory
Department of Mechanical Engineering University of Miami Coral Gables, Florida L. R. Lake P. J. Marto
Department of Mechanical Engineering Naval Postgraduate School Monterey, California This paper reports heat transfer measurements from the upper tube of an array of two horizontal, vertically aligned tubes immersed in a pool of CFC-114 at 1 atmosphere. Smooth and porous coated tubes were tested in separate arrays. The upper tube wall was instrumented with eight thermocouples in order to obtain an accurate average wall temperature. For the majority of tests, the heat flux on the lower tube was fixed while the heat flux on the upper tube was first increased up to 100 k W / m 2 and then decreased in order to obtain the familiar hysteresis loop. Tests were also conducted by varying the heat flux of both tubes by the same amount. Tube pitch to tube diameter ( P / D ) ratios of 1.5, 1.8 and 2 were tested. For both arrays, the influence of a lower tube on an upper tube in the natural convection region was small (up to 40%). For a lower boiling smooth tube, a factor of 6 increase in the heat transfer from an upper non-boiling
smooth tube was obtained at heat fluxes below 20 k W / m 2 and all evidence of boiling hysteresis was eliminated. This is attributed to impinging bubbles from below. At higher heat fluxes, with both smooth tubes boiling, the influence of bubbles from below was negligible. The only effect of a lower boiling porous coated tube was to promote slightly early partial nucleation. No other effect of the lower porous coated tube was found at any heat flux due to the high density of active nucleation sites present. However, boiling hysteresis was never completely eliminated. The influence of tube spacing was found to be small.
Effective Heat Exchange Surfaces for Vapor Generation G. A. Dreitser
Department of Theoretical Thermotechniks Moscow Aviation Institute Moscow, Russia I. Z. Kopp
Sankt-Petersburg Technic University Sankt-Petersburg, Russia This paper considers the experimental study of heat transfer for vapor generation on various liquids of different macro and microstructure surfaces. Our use of universal models for the main stages of the vapor generation mechanism (nucleation sites, bubble rise, and departure) enabled us to demonstrate the effect of the surface structure and properties on heat transfer. Good results were obtained to assist in determining the most advantageous conditions of using the effective heat exchange surfaces with assigned properties and structure: cleaning degree, special machining, presence of coatings, finning, etc. A general form of the solutions makes grounds for choosing optimal data on economizer and evaporation sections of heat exchange surfaces of vapor generators for liquid nucleate, transient or film boiling conditions.
Measurements of the Heat Transfer Coefficient and Pressure Drop in Flow Boiling of HCFC and HFC Refrigerants in Smooth and Microfinned Tubes G. Galli L. Laurenti F. Marcotullio A. Ponticiello
Dipartimento di Energetica Universith degli Studi di L'Aquila Localith Monteluco Roio Poggio (Aq), Italy The heat-transfer coefficient and pressure drop in forced flow boiling in a smooth tube and in two types of microfinned evaporators were measured for R12 and R134a. The test section was a straight, horizontal tube, heated by water circulated in a surrounding annulus. The evaporation temperature ranged from 5°C-15°C; the inlet and outlet qualities were 5 and 90%, respectively; and the specific mass flow rate ranged from 180-300 kh/m2s. The heat-transfer enhancement varies from 15%-40% in one of the microfinned tubes, and from 50%-70% in the other, when compared to the performance in a smooth tube in equivalent conditions. The increment in pressure drop for the first tube is not higher than 20% by comparison to the smooth tube, while the second microfinned tube shows an increment of about 100%.