Sheldon M. Jeter

An experimental investigation of single-phase forced convection in microchannels

Turbulent, single-phase forced convection of water in circular microchannels with diameters of 0.76 and 1.09 mm has been investigated. The data show that the Nusselt numbers for the microchannels are higher than those predicted by traditional large channel correlations. Based on the data obtained in this investigation, along with earlier data for smaller diameter channels, a generalized correlation for the Nusselt number for turbulent, single-phase, forced convection in circular microchannels has been developed. The diameter, Reynolds number, and Prandtl number ranges are 0.102–1.09 mm, 2.6 × 103−2.3 × 104, and 1.53–6.43, respectively. With a confidence level of greater than 95%, differences between experimental and predicted Nusselt number values are less than ± 18.6%.

Maximum conversion efficiency for the utilization of direct solar radiation

Abstract The maximum conversion efficiency for the utilization of direct solar energy is investigated. A fixed quantity of radiation is considered, and the essergy or potential work of the system is determined. The more significant case of a steady-flow process which is representative of realistic collection and utilization systems is also investigated. For the steady-flow process the Carnot efficiency for a reversible engine operating between fixed temperatures is obtained as the limiting value.

Calculation of the concentrated flux density distribution in parabolic trough collectors by a semifinite formulation

Abstract A first integral of the concentrated radiant flux density for trough concentrators is derived by exploiting the symmetry of these cylindrical systems. The resulting semifinite formulation yields both conceptual advantages and computational efficiency. The formulation is illustrated by application to the parabolic trough concentrator. The elementary example of determining the concentrated flux density at the nominal focal plane is first considered. The more realistic case of a round absorber tube, which can shade the reflector, centered on the focal line is also studied. This more practical geometry, which corresponds to most commercial designs, has previously received little attention in the open literature. These applications illustrate that the semifinite formulation presented herein can be a simple and useful tool for the analysis and design of trough concentrators.

Analytical determination of the optical performance of practical parabolic trough collectors from design data

Abstract A semifinite analytical formulation is presented to facilitate the efficient numerical computation of the concentrated radiant flux on receiver surfaces in the parabolic trough concentrator. The concentrated flux density is integrated to yield the interceptance ratio for the collector. By including models for the incident-angle dependent envelope transmittance and receiver absorptance, the concentration ratio for absorbed radiation is also calculated and integrated to yield the optical efficiency. Flux distribution results from this detailed analysis are compared with a point source model, and the optical efficiency results are compared with a first-order model. Since this model includes both a realistic nonuniform solar modeling as well as models for imperfect reflection, transmission, and absorption, it is capable of estimating the optical efficiency of practical collectors.

A critical heat flux correlation for droplet impact cooling

Abstract The characteristics of a single stream of monodispersed water droplets impacting a horizontal, upward facing flat surface have been investigated. The objective was to determine the effect of droplet diameter, impact frequency and impact velocity on the critical heat flux (CHF). A generalized correlation has been developed for the nondimensional CHF as a function of the Weber and Strouhal numbers of the impacting droplets. The Weber and Strouhal numbers ranged from 175 to 730 and 7.00 × 10 −3 −3.00 × 10 −2 , respectively. With a confidence level of greater than 95% differences between predicted and experimental CHF values were less than ±22%.

Gas-liquid two-phase flow in narrow horizontal annuli

Abstract Experimental data associated with the two-phase flow regimes, void fraction and pressure drop in horizontal, narrow, concentric annuli are presented. Two transparent test sections, one with inner and outer diameters of 6.6 and 8.6 mm, and an overall length of 46.0 cm; the other with 33.2 and 35.2 mm diameters and 43.0 cm length, respectively, were used. Near-atmospheric air and water constituted the gas and liquid phases, respectively. The gas and liquid superficial velocities were varied in the 0.02–57 and 0.1–6.1 m s −1 ranges, respectively. The major two-phase flow patterns observed included bubbly, slug/plug, churn, stratified, and annular. Transitional regimes, where the characteristics of two distinct flow regimes could be observed in the test sections, included bubbly-plug, stratified-slug and annular-slug. The obtained flow regime maps were different than flow regime maps typical of large horizontal channels and microchannels with circular cross-sections. They were also different from the flow regimes in rectangular thin channels. The measured average void fractions for the two test sections were compared with predictions of several empirical correlations. Overall, a correlation proposed by Butterworth [Butterworth, D., 1975. A comparison of some void fraction relationships for co-current gas–liquid flow. Int. J. Multiphase Flow 1, 845–850] based on the results of Lockhart and Martinelli (1949) provided the most accurate prediction of the measured void fractions. The measured pressure drops were compared with predictions of several empirical correlations. The correlation of Friedel [Friedel, L., 1979. Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow. 3R Int. 18, 485–492] was found to provide the best overall agreement with the data.

Onset of flow instability and critical heat flux in thin horizontal annuli

Abstract The onset of flow instability (OFI) and critical heat flux (CHF) in heated thin horizontal annular flow passages cooled by subcooled water were investigated. For OFI, six different test sections, with inner diameter of 6.4 mm, annular gap widths of 0.724–1.001 mm, and heated lengths of 174–197 mm were used. The test parameter ranges were: 85– 1428 kg/m 2 s mass flux, 0.344–1.034 MPa exit pressure, 50–150 °C inlet temperature, 0.124– 1.0 MW/m 2 surface heat flux and 0–∞ inner-to-outer surface heat flux ratio. In addition, the effect of dissolved non-condensables on OFI was examined by performing similar experiments with degassed water and water saturated with air at test section inlet temperature and exit pressure. A total of 138 OFI test were run addressing important parametric trends. A theoretical model based on the solution of one-dimensional fluid conservation equations, which assumes no voidage upstream the onset of significant void (OSV) point, and accounts for thermodynamic non-equilibrium beyond the OSV point using an empirical quality profile fit, was shown to predict the conditions leading to OFI reasonably well. For CHF, the test section was an annulus with 6.45 and 7.77 mm inner and outer diameters, respectively (0.66 mm gap width), and an 18.5-cm long heated section. The experimental parameters investigated covered the following ranges: test section exit pressure: 0.344–1.034 MPa; coolant (water) mass flux: 100– 380 kg/m 2 s ; wall heat flux: 0.231– 1.068 MW/m 2 ; water inlet temperature: 30–65 °C. The measured CHF values were considerably lower than the expected CHF values for vertical test section configuration. In all the tests CHF occurred at relatively high equilibrium qualities, and was preceded by flow stratification which caused dry-out of the upper surface of the flow channel. The data were correlated in two ways: by introducing empirical correction multipliers into three widely used correlations for vertical channels; and based on the compensated distortions method.

Modeling the transport processes within multichannel molten carbonate fuel cells

Increased concern over the environment degradation due to the emission of green house gases and interest in co-generating electricity and heat have stimulated intensive development of the fuel cell technology. In order to reduce the fuel cell manufacturing costs and to improve its performance, a better understanding of the fuel and oxidant species transport processes within fuel cells is critical. Fuel and oxidant flow distributions through fuel cell channels, usually through distribution manifold, have significant impact on fuel cell performance and efficiency. To this end, this paper presents the effects of the fuel and oxidant flow distributions on multi-channel fuel cell performance using a model of gas flow, heat and mass transfer with the electrochemical reactions for molten carbonate fuel cells.

Visualization of boiling bubble dynamics using a flat uniformly heated transparent surface

Bubble dynamics in saturated pool boiling of R-123 with and without an applied electric field have been investigated using a novel, flat, transparent heated surface. This method allows viewing and measurement of bubble dynamics from the entire heater surface without interference from the fluid or other bubbles. The data have been used to quantify the effect of an electric field on the latent heat contribution to the total heat flux and to demonstrate the effectiveness of this experimental technique. For a given heat flux, the application of the electric field reduces the surface temperature, thereby suppressing boiling and reducing the latent heat contribution.

Flow network analysis application in fuel cells

This paper deals with network flow analysis and its application to the calculation of flow distribution within fuel cells. The generated node and loop equations for the network analysis are presented. In order to accommodate changes in cell design, i.e. changes in flow network topology, the analysis includes automatic loop equation generation based on topology analysis and network search algorithm. The calculated flow distributions were in excellent agreement with experimental data.