Jet Flows from Bubbles During Subcooled Pool Boiling on Micro Wires

http://www.paper.edu.cn Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.4 385—402 385 Jet flows from bubbles during subcooled pool boiling on micro wires WANG Hao, D. M. Christopher, PENG Xiaofeng & WANG Buxuan Laboratory of Phase Change and Interfacial Transport Phenomena, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China Correspondence should be addressed to Peng Xiaofeng (email: [email protected]) Received January 4, 2005 Abstract An experimental investigation was conducted on subcooled nucleate boiling on ultra-small wires having diameters of 25―100 µm. High-speed photography and laser PIV (Particle Image Velocimetry) technology were used to visually observe the bubble dynamics. For highly subcooled boiling at moderate heat fluxes, the bubbles generally remained attached to the micro heating wires and bubble-top jet flows were clearly observed. Smaller bubbles usually had stronger bubble-top jet flows, while larger bubbles seemed to produce multi-jet flows. The structures of the bubble-top jet flows, as well as multi-jet flows, were proposed from the experimental observation. A model was developed to describe jet flow phenomena from bubbles on micro wires. Numerical simulations for bubbles having diameter of 0.03 and 0.06 mm showed that both the bubble-top and multi-jet flows were induced by a strong Marangoni effect due to high temperature gradients near the wire. The predicted velocity magnitudes and flow structures agreed very well with experimental measurements. The bubble size relative to the wire is an important factor affecting the jet flow structure. For a 0.03 mm bubble on a 0.1 mm wire, only a bubble-top jet flow forms, while a complex multi-jet flow pattern forms around the bubble with a weak bubble-top jet and two side jet flows for a 0.06 mm bubble. Keywords: subcooled boiling, bubble, multi-jet, jet flow, PIV, Marangoni, CFD. DOI: 10.1360/ 04ye0052/53 NOMENCLATURE Db, Vapor bubble diameter (m); Dw, heater wire diameter (m); hfg, latent heat (J/kg); hi, interface heat transfer 2 coefficient (W/m K); M , molecular weight of vapor (kg/kmol); R , universal gas constant (J/mol K); pl, liquid 2 2 2 3 pressure (N/m ); py, vapor pressure (N/m ); qw′′ , wire surface heat flux (W/m ); qw′′′ , wire volume heat flux (W/m ); 2 qi′′ , interfacial heat flux (W/m ); R, bubble radius (m); Tw, wire average temperature (K); Ti, liquid temperature at interface (K); Ts, saturated temperature under atmosphere (K); Tv, vapor temperature (K); v, velocity (m/s); λ, thermal conductivity (W/mK); β, liquid thermal expansivity (1/K); σ, surface tension coefficient (N/m); σˆ , accom- 3 3 modation coefficient; δ, liquid layer thickness (m); ρν, vapor density (kg/m ); ρl, liquid density (kg/m ); ν, kinetic viscosity (m2/s); µ, dynamic viscosity (Ns/m2). Subscripts: l, liquid; ν, vapor; I, interface; s, saturated. Copyright by Science in China Press 2005 转载中国科技论文在线 http://www.paper.edu.cn 386 Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.4 385—402 1 Introduction Nucleate boiling is widely encountered in a variety of practical applications, such as energy conversion, manufacturing process and chemical processing. However, because of their extreme complexity, many boiling phenomena still remained to be understood with no general theoretical models available to accurately predict boiling heat and mass transfer[1,2]. After his comprehensive review of the available investigations in the literature, Dhir[1] appealed to investigators to investigate boiling phenomena with renewed or unconventional ideas. In recent years, many investigators have paid more attention to the nonlinear interaction in boiling processes. Henley and Hummel[3] mentioned the interactions among nucleation sites. Sadasivan et al.[4―6] further indicated that available investigations nor- mally ignored the nonlinear characteristics of boiling processes, which is the most seri- ous shortcoming of conventional theories. Eddington and Kenning[7] conducted an investigation on the interaction among bubble productions occurring at adjacent sites. Kenning et al.[8,9] investigated the temperature field on a boiling surface using a liquid crystal layer on the bottom of the boiling surface and concluded that a comprehensive model for nucleate boiling ought to have the following features: Consideration of local superheats to determine the activity of sites and their contribution to the heat transfer; specification of sites by their properties of activation and cessation; accounting for the effect of intermittency of the overall heat flux. These investigations provide more fundamental understanding of nucleate boiling. However, most of these investigations have only qualitatively illustrated the nonlinear and dynamic behavior, lacking accurate mathematical descriptions of the interactions among nucleation sites during nucleation processes. Therefore, many unsolved problems and unrecognized phenomena still re- main. More recently, the development of digitally enhanced measurement and visualization techniques and the urgent need to predict the heat transfer for boiling in unconventional environments, especially microscale and microgravity environments, has resulted in more comprehensive investigations of boiling processes. Some interesting phenomena and dynamic processes were observed in the open literature. Lin et al.[10, 11] observed microscale homogeneous nucleation. Glod et al.[12] investigated the explosive vaporiza- tion of water close to its superheat limit at the microscale level. Nucleation jets and bubble-sweeping on micro wires were investigated in a sequence of experiments conducted by the authors’ group[13―15]. The PIV technique was used to visually observe flow fields around micro bubbles with intensive jet flows[16]. These studies have given an additional insight into the fundamental mechanisms controlling the boiling process and the complexity of nucleate boiling. Bubble-top jet flows, jet-like flows from the bubble top surface into the bulk liquid, are interesting phenomena observed in downward-facing subcooled boiling[17―19]. These jet flows represent important mechanisms in more accurate boiling heat transfer models, Copyright by Science in China Press 2005 中国科技论文在线 http://www.paper.edu.cn Jet flows from bubbles during subcooled pool boiling on micro wires 387 especially by clarifying the balance between microlayer evaporation and heat removal by the liquid phase, and both are widely recognized as key boiling heat transfer mechanisms. Many investigators sought to observe these phenomena and provide more experimental and theoretical evidence for better understanding of the physical nature of bubble-top jet flows. Various investigators suggested that the interfacial mass flux due to evaporation and condensation, Marangoni effect induced by the surface tension gradient and the surface pressure gradient resulting from evaporation all contribute to the jet flows. The present paper presents photographic and quantitative measurements data on bubble-top jet flows and multi-jet flows for subcooled pool boiling on ultrathin platinum wires. The structures of the bubble-top jet and multi-jet flows were proposed from the experimental observations and measurements. The various jet flow structures and the measured velocity profiles were independent of the orientation relative to gravity. A physical model was developed to describe the jet flow phenomena from bubbles on micro wires. 2 Experimental description The experimental facility employed in the present investigation consisted of three parts, the test section, the power supply and the high-speed photographic system, as shown in Fig. 1. The test section was a transparent vessel with a platinum heating wire inside the vessel. The wire could be placed horizontally or inclined. The platinum wires used in the experiments were 49 mm long having diameters of 0.1 mm or 0.025 mm. The photographic system included a high-speed CCD camera, a high-resolution image acquisition card, and zoom lenses. The frame speed was 30―1000 frames per second. The power supply provided direct current to the platinum wire for Joule heating. The ends of the wires were connected tightly to copper posts. The pressure in the vessel was kept at atmospheric pressure. The current and voltage to the platinum wire were measured to determine the input power and the wire resistance. The average wire temperature was estimated using a calibrated resistance-temperature correlation. The bulk liquid temperature was measured using thermocouples and ther- mometers placed in the bulk liquid. The back lighting with some angle inclined to the wire was used. Detailed measurements of the jet flow field were obtained using a 2-D particle image velocimetry (PIV) system. The PIV system includes an imaging subsystem, an image capture subsystem and an analysis and display subsystem. The PIV system and experimental facility are illustrated in Fig. 2 and Fig. 3. Because the bubbles on the heater wire were very small (the heater wires were 0.1 or 0.025 mm in diameter), the CCD camera was equipped with a series of zoom lens. 1 µm aluminum particles were used as tracing particles. www.scichina.com 中国科技论文在线 http://www.paper.edu.cn 388 Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.4 385—402 Fig. 1. Experiment setup. Fig. 2. PIV system. Fig. 3. PIV experimental equipment. 3 Experimental observations 3.1 Bubble-top jet flows For highly subcooled boiling at moderate heat fluxes, the bubbles generally remained attached to the micro heating wire for a long time, with clearly observed bubble-top jet flows. Fig. 4 shows several well-developed bubble-top jet flows recorded with the CCD camera. More detailed information was obtained from the PIV measurements, as shown in Fig. 5 in which the arrows represent the direction and magnitude of the local velocity vectors. The left picture in Fig. 5 is the real photo superimposed with the flow field vectors. The bubble-top jet flows had a stable structure and intensity which pumped liquid into the bulk region.

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