Michael K. Jensen

ANALYSIS OF HEAT TRANSFER CHARACTERISTICS IN INTERNALLY FINNED TUBES

An in-depth physical analysis of turbulent flow and heat transfer mechanisms in internally finned tubes is presented. For a trapezoidal fin shape, the numerical analysis produced a series of full flow visualizations to clarify the governing mechanisms in fin tube flow. The parametric study explored the effects of number of fins N, nondimensional fin height H, helix angle γ, and fin width s on both friction and heat transfer. Three representative fin tube geometries in the range of , and , were studied. In the literature, the governing physics of tall fin and microfin tubes are believed to be different. However, the current visualization results suggest that there is a continuum in the governing flow physics regardless of fin geometry.

Critical heat flux in helical coils with a circumferential heat flux tilt toward the outside surface

Abstract A study of the forced-convection boiling of R-113 in electrically heated coils with a substantial heat flux tilt toward the outside of the coils is reported. The circumferentially local subcooled critical heat flux (CHF) with a flux tilt is increased or decreased, when compared to a coil without a flux tilt at the same bulk fluid condition, depending on the relative magnitudes of the flux tilt, mass velocity and ratio of tube diameter to coil diameter ( d / D ). Generally, there are increases in the local CHF with decreasing mass velocity and d / D and with increasing flux tilt. The quality region CHF in a coil with a flux tilt is lower than the CHF in a coil without a flux tilt. The subcooled and quality region CHF condition (when unaffected by buoyancy) occurs at the inside surface of the coil and not at the location of highest heat flux. Operational problems, in particular upstream CHF conditions, may occur if a coiled tube is operated with low to moderate subcooling near the inlet and with moderately high heat fluxes.

COMBINED CONVECTION AND RADIATION IN SIMULTANEOUSLY DEVELOPING FLOW AND HEAT TRANSFER WITH NONGRAY GAS MIXTURES

Abstract Combined convection and radiation in simultaneously developing laminar flow and heat transfer in a smooth tube is numerically considered with the P-I approximation and the exponential wideband model. The fluid is a mixture of carbon dioxide, water vapor, and nitrogen, The bulk mean temperature variation, temperature profiles, and Nusselt numbers are shown for a uniform inlet temperature and several constant wall temperatures. Nusselt numbers for simultaneously developing flow are compared with those for thermally developing flow. The effects of the mole fraction of the nongray gases and the wall emissivity on convective and radiative Nusselt numbers are explored.

SIMULATION OF THREE-DIMENSIONAL INCOMPRESSIBLE TURBULENT FLOW INSIDE TUBES WITH HELICAL FINS

A stabilized finite-element solver is used to solve turbulent flows and heat transfer in complex, three-dimensional geometries. Selected turbulence models are implemented and evaluated for their performance. Particular attention is given to the initial conditions for k and ε necessary to facilitate convergence. Details of the stabilized finite-element formulations for incompressible flow are also presented, and cost-effective model simplifications are introduced. For the analysis of heat transfer, an efficient technique for solving the energy equation without introducing a nondimensional temperature equation is also presented. As a test case, tubes with helical fins are selected.

The Critical Heat Flux Condition With Water in a Uniformly Heated Microtube

The critical heat flux (CHF) condition needs to be well understood for designing miniature devices involving two-phase flow. Experiments were performed to determine the CHF condition for a single stainless steel tube having an inside diameter of 0.427 mm subjected to uniform heat flux boundary conditions. The effects of mass flux, pressure, and exit quality on the CHF were investigated. The experimental results show that the CHF increases with an increase in mass flux and exit pressure. For all exit pressures, the CHF decreased with an increase in quality in the subcooled region, but with a further increase in quality (near zero quality and above), the CHF was found to have an increasing trend with quality (up to about 25% quality). CHF values in this region were much higher than those in the subcooled region. This suggests that even at very low qualities, the void fraction becomes appreciable, which results in an increase in the average velocity, thereby increasing the CHF limit.

Two-phase pressure drop with twisted-tape swirl generators

Abstract An experimental study has been conducted to determine the effect of twisted-tape swirl generators on adiabatic and diabatic two-phase flow pressure drops in vertical straight tubes. Tape-twist ratios (length for 180° twist/inside tube diameter) of 3.94, 8.94, and 13.92 were tested with R-113 over a range of pressures, mass velocities, qualities, and heat fluxes. Empty tube refcrence data were successfully predicted with a correlation from the literature. The twisted tape data were successfully correlated by using the hydraulic diameter and a single-phase swirl flow friction factor in the empty tube correlation. Data from the literature also were predicted well with this correlation.

An evaluation of the effect of twisted-tape swirl generators in two-phase flow heat exchangers

Correlations covering all modes of single- and two-phase heat transfer with twisted-tape swirl generators have been incorporated into a computer program that predicts the effect of twisted tapes on the heat duty in fluid-to-fluid once-through vapor generators. These Results are compared with those from axial flow once-through vapor generators operating at the same inlet conditions. Pumping power and pressure drop with and without swirl generators are compared. The correlations used satisfactorily predict the performance of both types of heat exchangers. Significant area, pressure drop, and pumping power reductions are possible using twisted-tape swirl generators; almost the entire area reduction is attributable to the suppression of the critical heat flux condition.