James F. Klausner

Vapor bubble departure in forced convection boiling

Abstract A forced convection boiling facility has been fabricated in which vapor bubble departure can be investigated. It has been observed that once a vapor bubble departs from its nucleation site, it typically slides along the heating surface and lifts off at some finite distance downstream. The probability density functions (pdfs) for bubble departure diameter, d , have been obtained for mass flux, C , ranging from 112 to 287 kg m −2 s −1 and heat flux, q w , ranging from 11.0 to 26.0 kW m −2 . The data indicate a systematic dependence of d on G and q w . A detailed analysis of various forces acting on the bubble is presented and is used to predict the mean departure diameter. The onset of imbalance between the quasi-steady drag, the unsteady component of the drag due to asymmetrical bubble growth, and the surface tension force in the flow direction is used as a criterion for departure and yields satisfactory agreement between the measured and predicted values of the mean departure diameter. The analytical prediction shows a strong influence of mean liquid velocity and wall superheat on the bubble departure diameter. At the point of departure the surface tension force in the flow direction is generally small.

A unified model for the prediction of bubble detachment diameters in boiling systems-II. Flow boiling

An improved model is proposed for the prediction of departure and lift-off diameters in saturated forced convection boiling. The model utilizes a force balance similar to that proposed by Klausner et al. (Int. J. Heat Mass Transfer 36, 651–662 (1993)). One significant improvement is that the inclination angle is determined on a dynamic basis and is not required as an input. Furthermore, it is hypothesized that the surface tension force is small compared to other forces acting on a vapor bubble at the points of departure and lift-off, and thus information on the bubble contact diameter and contact angles is not required. A new data set on mean vapor bubble lift-off diameters and probability density functions (pdfs) for flow boiling of refrigerant R113 on a nichrome heating strip has been obtained using the experimental facility described by Klausner et al. (Int. J. Heat Mass Transfer 36, 651–662 (1993)). The wall superheat and mean liquid velocity respectively range from 5.5 to 12.0 °C and 0.35 to 1.0 m s−1. It is demonstrated that over the limited range of flow boiling conditions considered, the predicted departure and lift-off diameters agree well with measured values.

A NOTE ON THE HISTORY FORCE ON A SPHERICAL BUBBLE AT FINITE REYNOLDS NUMBER

An approximate expression for the history force on a spherical bubble is proposed for finite Reynolds number, Re. At small time, the history‐force kernel is a constant, which decreases with increasing Re, and the kernel decays as t−2 for large time. For an impulsively started flow over a bubble, accurate finite difference results show that the history force on the bubble decays as t−2 at large time. Satisfactory agreement is observed between the presently proposed history force and the numerical solution.

Shear lift force on spherical bubbles

Abstract The shear lift force on a spherical bubble in an unbounded shear flow at low Reynolds number is derived. It is two thirds of that for a solid sphere. An approximate expression for the shear lift force at finite Reynolds number and finite shear rate is obtained by an interpolation using the present result and Autons result at large Re and small shear rate.

Decreased imposed work with a new nasal continuous positive airway pressure device

This model study compared the imposed work of two nasal continuous positive airway pressure (NCPAP) devices for very low birthweight (VLBW) babies; a new NCPAP device, designed by Moa et al., (Crit Care Med 16:1238, 1988), and a conventional NCPAP device. In addition, the variabilities in the pressures of a simulated airway were compared. A continuous flow (8 L/min) was used to generate CPAP. A Harvard ventilator was used to simulate breathing at a fixed volume (12.1 mL) and rate (45 breaths/min). The pressure drop across and the flow rate through each device were measured, thus allowing the determination of imposed work, i.e., the work of breathing done by the patient to overcome frictional losses due to the NCPAP device. In addition, the pressure fluctuations in the simulated airway were measured. The data presented are averages of 36 breaths using the new device and 34 breaths using the conventional device. The means of the imposed work in the new device and in the conventional device were 0.135 (95% Cl ± 0.004) mJ/breath and 0.510 (95% Cl ± 0.004) mJ/breath (P < 0.01) respectively. The coefficients of variation for pressure in the simulated airway were: new 6.8% and conventional 15.3%. We conclude that the imposed work of the new NCPAP device for the VLBW baby is approximately one‐fourth of that of the conventional device. The airway pressure generated by the VLBW size of the new NCPAP device shows less variability during simulated breathing than that found with the conventional device. The findings support the clinical use of the new NCPAP device in the very low birthweight baby. Pediatr Pulmonol. 1996; 22:188–194.

Vapor bubble growth in heterogeneous boiling—I. Formulation

Abstract A numerical analysis is carried out to study bubble growth in saturated heterogeneous boiling. The bubble growth is determined by considering the simultaneous energy transfer among the vapor bubble, liquid microlayer, and heater. Finite difference solutions for the temperature fields in the microlayer and heater are obtained on expanding coordinates as the bubble grows. The parameters characterizing the bubble shape and microlayer wedge angle are determined by matching the existing experimental data. The predicted bubble growth rate compares very well with the reported experimental data over a wide range of conditions.

Unsteady force on a spherical bubble at finite Reynolds number with small fluctuations in the free‐stream velocity

Unsteady flow over a stationary spherical bubble with small fluctuations in the free‐stream velocity is considered for Reynolds number ranging from 0.1 to 200. Solutions to the Navier–Stokes equations of both steady and unsteady components are obtained using a finite‐difference method and a regular perturbation scheme based on the amplitude of the fluctuations being small. The dependence of the unsteady drag on the frequency of the fluctuations is examined at finite Reynolds number. It is shown that the quasisteady drag can be represented by using the steady‐state drag coefficient and the instantaneous velocity. Numerical results indicate that the unsteady force at low frequency, ω, increases linearly with ω rather than increasing linearly with ω1/2, which results from the creeping flow solution of the Stokes equation. The added‐mass force at finite Reynolds number is found to be the same as in creeping flow and potential flow. The history force at finite Re is identified and carefully evaluated. The imaginary component of the history force increases linearly with ω when ω is small and decays as ω−1/2 as ω becomes large. The implication is that the history force has a much shorter memory in the time domain than predicted by the solution of the unsteady Stokes equation. Numerical results suggest that the history force, which is due to the combination of the viscous diffusion of the vorticity and the acceleration of the flow field, at low frequency is finite even at large Reynolds number.

Performance of Nonconcentrating Solar Photocatalytic Oxidation Reactors: Part I—Flat-Plate Configuration

A nonconcentrating flat-plate photoreactor has been fabricated and tested in an outdoor solar photocatalytic oxidation facility. It has been demonstrated that 4-chlorophenol (4CP) is successfully oxidized when operating under either sunny or cloudy atmospheric conditions. Tests have been conducted with the photocatalyst, titanium dioxide (TiO[sub 2]), either mixed into the 4CP solution to form a slurry or adhered to a fiberglass mesh in a fixed configuration. The reaction rate constant for the slurry mode is typically two to five times greater than that for the fixed catalyst mesh tested at similar ultraviolet (UV) insolation conditions. In addition, the reaction rate constant appears to vary linearly with the UV insolation, and it shows no dependence on liquid film thickness in the slurry mode, but appears to vary linearly with the inverse of film thickness in the fixed catalyst mode. All tests were performed in the laminar flow regime. Design recommendations for industrial development are presented.

The Influence of Vapor Bubble Sliding on Forced Convection Boiling Heat Transfer

We describe experimental efforts aimed at examining the effect of vapor bubble sliding on forced convection boiling heat transfer. Flow boiling experiments using FC-87 were conducted for vertical upflow and downflow configurations. Both slightly subcooled single-phase and saturated annular flow boiling were considered. Significantly higher heat transfer rates were measured for vertical upflow than for downflow with the same wall superheat, and slightly subcooled single-phase inlet conditions. This increase in heat transfer is directly attributable to sliding vapor bubbles, which remain attached to the wall during upflow and lift off the wall during downflow. Differences in the measured upflow and downflow heat transfer rates are not as significant for annular flow boiling, which is clue in part to the similar vapor bubble dynamics which have been observed for upflow and downflow. Heat transfer experiments in single-phase subcooled upflow with air bubble injection at the heating surface suggest that sliding bubbles enhance the bulk liquid turbulence at the wall, which contributes significantly to the macroscale heat transfer

A Flow Boiling Microchannel Evaporator Plate for Fuel Cell Thermal Management

In order to provide a high-power density thermal management system for PEM fuel cell applications, a flow boiling microchannel evaporator plate has been developed that operates in a closed loop two-phase thermosyphon. The flow is passively driven by gravity, and the flow rate initially increases with increasing evaporation rate and then decreases after reaching a peak flow rate. A microchannel plate has been fabricated with 56 square channels that have a 1 mm × 1 mm cross-section and are 115 mm long. The working fluid, HFE-7100, has been chosen due to its favorable saturation temperature at one atmosphere. Experiments have been conducted with the heat flux as the control variable. Measurements of mass flow rate, temperature field, and pressure drop have been made. The flow regimes are predominately bubbly and slug. The maximum heat flux observed, 32 kW/m2, is an order of magnitude greater than that required in current fuel cells and is limited by a Ledinegg instability. Two-phase thermal hydraulic models give a reasonable prediction for the mass flow rates and wall temperatures using standard flow boiling correlations. This paper will thoroughly describe the performance of the two-phase thermal management system over a wide range of operating conditions.