Finite Heater Research Articles

A modified hydrodynamic model for pool boiling CHF considering the effects of heater size and nucleation site density

Boiling is commonly used in daily life and is an efficient heat removal method. During pool boiling, the critical heat flux (CHF) establishes the upper limit of efficient heat removal. Although, numerous studies have explored CHF, its exact mechanisms remain ambiguous. The hydrodynamic instability model is the most prominent CHF model, but it fails to explain the evident dependence of CHF on surface properties. The authors proposed modifying the hydrodynamic model to account for how nucleation site density and finite heater size affect the CHF. The experimental CHF values obtained from evaluating various types of surfaces qualitatively agreed with the predictions of the modified hydrodynamic model. This suggests that the CHFs on these surfaces were due to the hydrodynamic limit.

Bifurcation in a thin liquid film flowing over a locally heated surface

AbstractWe investigate the nonlinear dynamics of a two-dimensional film flowing down a finite heater, for a non-volatile and a volatile liquid. An oscillatory instability is predicted beyond a critical value of the Marangoni number using linear stability theory. Continuation along the Marangoni number using a nonlinear evolution equation is employed to trace the bifurcation diagram associated with the oscillatory instability. Hysteresis, a characteristic attribute of a subcritical Hopf bifurcation, is observed in a critical parametric region. The bifurcation is universally observed for both a non-volatile film and a volatile film.

An experiment on the dynamics of thermal diffusion

We present an experiment that demonstrates thermal diffusion in metals via a dynamic measurement of the temperature in a metal rod as a function of position and time. From this single measurement and using simple heat flow equations, we can extract the thermal conductivity and the specific heat of the metal to within 5% of the accepted values. This experiment can be extended for advanced students, who can model the heat flow by including heat losses and finite heater models via a numerical solution of a partial differential equation.

Analytical Solution of the Heat Pulse Method in a Parallelepiped Sample Space

The heat pulse method enables estimation of the thermal diffusivity (k), volumetric heat capacity (C), and thermal conductivity of soils (λ) and soil water content. In this study, analytical solutions were derived by the method of Green's function for a finite pulse cylinder source in a parallelepiped sample of size a by b by c with two kinds of boundary conditions. One is the zero surface temperature (ZST) boundary condition, and the other is the adiabatic boundary condition (ABC). The proposed solutions may be useful for evaluating the errors of a dual‐probe heat pulse (DPHP) system introduced by approximating the finite heater with an infinite line source (ILS) model. Applications of the solution are presented in the context of an air‐dried virtual soil sample to demonstrate how different factors (boundary conditions, soil sample size, heater needle length and radius, probe spacing, heating duration, and strength) can affect the error in k and C caused by using an ILS model. For a given parallelepiped (or soil column), the larger the ratio of lengths of the heater probe and the sample, the smaller the boundary influence on the temperature rise at the mid‐needle temperature sensing location, and the smaller the errors introduced by using the ILS approximation. For various heating strengths, it was found that the errors in both k and C were relatively constant when all other parameters were fixed. These errors increased monotonically and slowly, however, as heat duration increased.

Numerical investigation of three-dimensional active control of boundary-layer transition

A numerical model is developed for the investigation of boundary-layer transition control of spatially evolving instability waves. Active control of a periodically forced boundary layer in an incompressible fluid is simulated using surface beating techniques. The Navier-Stokes and energy equations are solved using a fully implicit finite-difference/spectral method. Temperature perturbations are introduced locally along finite heater strips to directly attenuate instability waves in the flow. A Feedback control loop is employed in which a downstream sensor is used to monitor wall shear stress fluctuations

Sidewall and Immersion-Depth Effects on Pool Boiling Burnout for Horizontal Cylindrical Heaters

The horizontal cylinder is the geometry in which the problem of evaluating the peak nucleate boiling heat flux q{sub max} (commonly called the burnout heat flux) has been most studied. Countless data have been presented but many are subject. Countless data have been presented but many are subject to a variety of nuisance variables. The objective of the present work is to identify experimentally the range of influence of two of these nuisance variables: sidewall blockage and immersion depth. The existing hydrodynamic predictions of q{sub max} envision either an infinite flat plate beneath an infinitely deep liquid bath, or a finite heater immersed in a bath of infinite extent. Attention has been given to such effects in other geometries. However, the effect of nearby walls or depth of immersion on horizontal cylinder has not been systematically documented. The authors aim here is determine, experimentally, how q{sub max} is altered when a long horizontal cylinder is centered between vertical sidewalls separated by a distance and located a distance below the liquid surface.

Hydrodynamic Prediction of Peak Pool-boiling Heat Fluxes from Finite Bodies

Since Zuber made a hydrodynamic prediction of the peak pool-boiling heat flux on an infinite flat plate, his general concept has been used to predict the peak heat flux in two finite heater configurations. These latter predictions have differed from Zuber’s in the introduction of a largely empirical variable—the thickness of the vapor escape path around the body. The present study shows how measurements of this thickness can be combined with the hypothesis that the vapor velocity within the vapor blanket must match the vapor velocity in the escaping jet above the heater. The result is a more exact description of the hydrodynamics of vapor removal. This idea is used to suggest the possibility of a universal value for the ratio of the cross-sectional area of escaping jets to the heater area for large finite heaters and for long slender heaters. A set of general ground rules is developed for predicting the peak heat fluxes on both large and small heaters. These rules are used in turn to predict the peak heat flux from horizontal ribbons. They are also used to correct the traditional prediction for infinite-flat-plate heaters. The predictions are supported with new data.