Saturated Flow Boiling Research Articles

Local Heat Transfer of Saturated Flow Boiling in Vertical Narrow Microchannel

Abstract An experimental study of saturated flow boiling in a high-aspect-ratio one-side-heating rectangular microchannel was conducted with de-ionized water as the working fluid. ZnO microrods with the average diameter of about 1 μm and length of about 7 μm were synthesized on the Ti wafer surface, which was used to fabricate the heated bottom surface of the microchannel. The ZnO microrod surface appeared to be hydrophobic and the capillary wetting effect on the surface was found after being wet. The heat transfer and pressure drop characteristics of saturated flow boiling in the microchannel were studied and the flow patterns were photographed with a high-speed camera. Almost all the flow patterns observed in this experiment featured the main annular flow and abrupt flush of bubbly flow. Because of the capillary wetting effect on the ZnO microrod surface, the local dryout and rewetting phenomenon did not appear in this study. However, due to the numerous nucleation sites on ZnO microrod surface, the abrupt bubble flow caused much more disruption to the liquid film of annular flow when compared to the regular silicon surface. The abrupt bubble flow flushed through the annular liquid film and caused the fluctuation and nonuniformity of the liquid film and heat transfer deterioration, which was severer in the high heat flux conditions. Otherwise, the capillary effect on the ZnO microrod surface was able to restrict the nonuniformity of the liquid film under high heat flux and low mass flux conditions; thus, the deterioration of heat transfer performances diminished.

Numerical Investigation of Saturated Flow Boiling in Microchannels by the Lattice Boltzmann Method

Bubble formation in saturated flow boiling in 2D microchannels, generated from a microheater under constant wall heat flux or constant wall temperature conditions, is studied numerically based on a newly developed lattice Boltzmann model for liquid-vapor phase change. Simulations are carried out to study effects of inlet velocity, contact angle, and heater size on saturated flow boiling of water under constant wall heat flux conditions. Important information, such as effects of static contact angle on nucleation time and nucleation temperature, which was unable to be obtained by other numerical simulation methods, is obtained. Furthermore, effects of inlet velocity, contact angle, and superheat on nucleate boiling heat transfer in steady flow boiling of water under constant wall temperature conditions are also presented. It is found that the nucleate boiling heat transfer at the microheater is higher if the heater surface is more hydrophilic, because the superheated vapor at the hydrophilic wall has a thinner thermal boundary layer and a larger thermal conductivity.

A Scale Analysis Based Theoretical Force Balance Model for Critical Heat Flux (CHF) During Saturated Flow Boiling in Microchannels and Minichannels

Accurate prediction of critical heat flux (CHF) in microchannels and minichannels is of great interest in estimating the safe operational limits of cooling systems employing flow boiling. Scale analysis is applied to identify the relevant forces leading to the CHF condition. Using these forces, a local parameter model is developed to predict the flow boiling CHF. The theoretical model is an extension of an earlier pool boiling CHF model and incorporates force balance among the evaporation momentum, surface tension, inertia, and viscous forces. Weber number, capillary number, and a new nondimensional group introduced earlier by Kandlikar (2004, “Heat Transfer Mechanisms During Flow Boiling in Microchannels,” ASME J. Heat Transfer, 126, pp. 8–16), K2, representing the ratio of evaporation momentum to surface tension forces, emerged as main groups in quantifying the narrow channel effects on CHF. The constants in the model were calculated from the available experimental data. The mean error with ten data sets is 19.7% with 76% data falling within ±30% error band and 93% within ±50% error band. The length to diameter ratio emerged as a parameter indicating a stepwise regime change. The success of the model indicates that flow boiling CHF can be modeled as a local phenomenon and the scale analysis is able to reveal important information regarding fundamental mechanisms leading to the CHF condition.

Surface Roughness Effects on Flow Boiling in Microchannels

The influence of surface roughness on flow boiling heat transfer and pressure drop in microchannels is experimentally explored. The microchannel heat sink employed in the study consists of ten parallel, 25.4 mm long channels with nominal dimensions of 500×500 μm2. The channels were produced by saw-cutting. Two of the test piece surfaces were roughened to varying degrees with electrical discharge machining (EDM). The roughness average Ra varied from 1.4 μm for the as-fabricated, saw-cut surface to 3.9 μm and 6.7 μm for the two roughened EDM surfaces. Deionized water was used as the working fluid. The experiments indicate that the surface roughness has little influence on boiling incipience and only a minor impact on saturated boiling heat transfer coefficients at lower heat fluxes. For wall heat fluxes above 1500 kW/m2, the two EDM surfaces (3.9 μm and 6.7 μm) have similar heat transfer coefficients that were 20–35% higher than those measured for the saw-cut surface (1.4 μm). A modified Bertsch et al. [2009, “A Composite Heat Transfer Correlation for Saturated Flow Boiling in Small Channels,” Int. J. Heat Mass Transfer, 52, pp. 2110–2118] correlation was found to provide acceptable predictions of the flow boiling heat transfer coefficient over the range of conditions tested. Analysis of the pressure drop measurements indicates that only the roughest surface (6.7 μm) has an adverse effect on the two-phase pressure drop.

Review and Comparative Analysis of Studies on Saturated Flow Boiling in Small Channels

Recent studies relating to saturated flow boiling in minichannels and microchannels are reviewed, with a focus on the functional dependence of the heat transfer characteristics on thermodynamic vapor quality. While there is general agreement in the literature that the heat transfer coefficient increases with increases in heat flux, conflicting trends have been reported on the influence of vapor quality. A compilation and analysis of the results from recent investigations in the literature on flow boiling in small channels is presented. Further, correlations for flow boiling proposed in these studies are quantitatively assessed by comparing them against 10 independent data sets from the published literature covering a range of hydraulic diameters from 0.16 to 2.01 mm. Recommended topics for future research in this field are also identified. [Supplemental materials are available for this article. Go to the publisher's online edition of Nanoscale and Microscale Thermophysical Engineering for the following free supplemental resources: an appendix containing heat transfer correlations and a database collected from the literature for flow boiling heat transfer in small channels.]

Thermodynamics of Void Fraction in Saturated Flow Boiling

AbstractRecently, Collado (Proc, IMECE 2001, Symposium on Fluid Physics and Heat Transfer for Macro- and Micro-Scale Gas-Liquid and Phase Change Flows) suggested calculating void fraction, an essential element in thermal-hydraulics, working with the “thermodynamic” quality instead of the usual “flow” quality. The “thermodynamic” quality is a state variable, which has a direct relation with the actual vapor volumetric fraction, or void fraction, through phase densities. This approach provides a procedure for predicting void fraction, if values of “thermodynamic” quality are available. However, the standard heat balance is usually stated as a function of the “flow” quality. Therefore, we should search for a new heat balance between the mixture enthalpy, based on “thermodynamic” quality, and the absorbed heat. This paper presents the results of such analysis based on the accurate measurements of the outlet void fraction measured during the Cambridge project by Knights (1960, “A Study of Two-Phase Pressure Drop and Density Determination in a High-Pressure Steam-Water Circuit,” Ph.D. thesis, Cambridge University Engineering Lab, Cambridge, UK) in the 1960s for saturated flow boiling. In the 286 tests analyzed, the pressure and mass fluxes range from 1.72 MPa to 14.48 MPa and from 561.4 to 1833.33 kgm−2s−1, respectively. As the main result, we find that the slip ratio would close this new thermodynamic heat balance. This has allowed the accurate calculation of void fraction from this balance, provided we can predict the slip ratio. Finally, the strong connection of this new thermodynamic heat balance with the standard one through the slip ratio is highlighted.

Saturated Flow Boiling of Water in a Narrow Channel: Experimental Investigation of Local Phenomena

This paper describes some observations of flow boiling of water at near-atmospheric pressure in a single vertical channel of rectangular cross-section 2 mm × l mm, heated on three sides over a length of 248 mm, with a window on the fourth side. Measurements have been made of the fluctuating wall temperatures and pressures at several stations along the channel, with simultaneous high-speed video recordings of the flow patterns at a mass flux of 134 kg m −2 s −1 and heat fluxes of 50–100 kW m −2 . The frequency spectra of the fluctuations depend on the compressibility of the inlet flow to the channel. It is shown that the processes of initiation of boiling and of heat transfer differ from flow boiling in large channels. The acceleration of liquid slugs by confined bubbles causes pressure pulses to propagate the full length of the channel. Heat transfer is distributed in time between convection and nucleate boiling. The experimental limitations are discussed.

Development of a Flow Boiling Map for Subcooled and Saturated Flow Boiling of Different Fluids Inside Circular Tubes

The thermal behavior of a flow boiling system is represented by a flow boiling map to illustrate visually the relationships among various system parameters. An earlier flow boiling map by Collier (1981) does not include the effect of mass flux and is specific to water at low pressures. For other fluids, significant departures from the parametric trends displayed in Collier’s map have been reported in the literature (e.g., Kandlikar, 1988b). In the present paper, a new flow boiling map is developed to depict the relationships among the heat transfer coefficient, quality, heat flux, and mass flux for different fluids in the subcooled and the saturated flow boiling regions. The trends observed in the experimental data and correlations for water and refrigerants are used in deriving the present map. The particular areas where further investigation is needed to validate the trends are also indicated. In the subcooled boiling region, hTP/hlo is plotted against x with Bo as a parameter, while in the saturated boiling region, hTP/hlo is plotted against x with ρl/ρg and a modified boiling number Bo* as parameters. It is hoped that the map would prove to be helpful in explaining the role of different heat transfer mechanisms in flow boiling of different fluids.