Minimum Film Boiling Heat Flux Research Articles

Parametric investigation on transient boiling heat transfer of metal rod cooled rapidly in water pool

In this work, the effects of liquid subcooling, surface coating, material property, and surface oxidation on transient pool boiling heat transfer were investigated experimentally using the vertical metal rod and quenching method. The change in rod temperature was measured with time during quenching, and the visualization of boiling around the test specimen was performed using the high-speed video camera. As the test materials, the zircaloy (Zry), stainless steel (SS), niobium (Nb), and copper (Cu) were tested. In addition, the chromium-coated niobium (Cr-Nb) and chromium-coated stainless steel (Cr-SS) were prepared for accident tolerant fuel (ATF) application. Low liquid subcooling and Cr-coating shifted the quenching curve to the right, which indicates a prolongation of quenching duration. On the other hand, the material with small heat capacity and surface oxidation caused the quenching curve to move to the left. To examine the influence of the material property and surface oxidation on the film boiling heat transfer performance and minimum film boiling (MFB) point in more detail, the wall temperature and heat flux were calculated from the present transient temperature profile using the inverse heat transfer analysis, and then the curves of wall temperature and heat flux in the film boiling regime were obtained. In the present experimental conditions, the effect of material property on the film boiling heat transfer performance and MFB point seemed to be minor. On the other hand, based on the experimental results of the Cu test specimen, the surface oxidation had great impact on them. As compared with the as-received Cu test specimen, the pre-oxidized one showed the higher film boiling heat transfer performance and MFB heat flux, but it had the lower MFB temperature.

Experiments on Pool Boiling of Water from Downward-Facing Hemispheres

Quenching experiments were conducted to investigate pool boiling of saturated water on downward-facing aluminum and 303e stainless steel hemispheres. Test sections had an outer diameter of 0.152 m and a wall thickness of 0.020 m. Destabilization of film boiling and wetting of the stainless steel surface occurred earlier than with aluminum (15 s versus 92 s), at 20 K higher wall superheat and ~10% higher minimum film boiling heat flux qmin. Wetting of the stainless steel surface occurred first near the edge of the test section and then gradually propagated azimuthally inward, followed by the maximum heat flux (MHF) front. Conversely, wetting of the aluminum surface occurred first at the lowermost position (θ = 0 deg) and then propagated azimuthally outward. The azimuthal propagation of the MHF front on the stainless steel surface (~5 deg/s for 60 deg < θ < 90 deg decreasing to ~1.8 deg/s for 10 deg < θ < 60 deg and then increasing slightly to ~2 deg/s for 0 deg < θ < 60 deg) was much slower than on aluminum (~22.5 deg/s on average). The MHF front traversed the entire stainless steel boiling surface in ~40 s versus only 4 s for aluminum. When MHF for the latter first occurred at θ = 0 deg, the radial and near-boiling surface azimuthal temperature gradients were 4 K/mm and 0.6 K/deg, respectively, compared to 12 to 15 K/mm and 1.5 K/deg for stainless steel. For both surfaces, the MHF and qmin values displayed parabolic dependencies on azimuthal angle. The local MHF at θ = 0 deg was 0.81 and 0.40 MW/m2 for aluminum and stainless steel, respectively, decreasing with increased azimuthal angle to minimums of 0.47 and 0.256 MW/m2 at θ = 45 deg. Beyond 45 deg, the local MHF increased with increased azimuthal angle to 0.76 and 0.47 MW/m2, respectively, near the edge of the surface (θ = 80 deg). However, the wall superheats corresponding to the MHF (30 K for aluminum and 80 K for stainless steel) and the qmin (125 K for aluminum and 145 K for stainless steel) were independent of azimuthal angle.

An investigation of transition boiling mechanisms of subcooled water under forced convective conditions

A mechanistic model for forced convective transition boiling has been developed to investigate transition boiling mechanisms and to predict transition boiling heat flux realistically. This model is based on a postulated multi-stage boiling process occurring during the passage time of the elongated vapor blanket specified at a critical heat flux (CHF) condition. Between the departure from nucleate boiling (DNB) and the departure from film boiling (DFB) points, the boiling heat transfer is established through three boiling stages, namely, the macrolayer evaporation and dryout governed by nucleate boiling in a thin liquid film and the unstable film boiling characterized by the frequent touches of the interface and the heated wall. The total heat transfer rates after the DNB is weighted by the time fractions of each stage, which are defined as the ratio of each stage duration to the vapor blanket passage time. The model predictions are compared with some available experimental transition boiling data. The parametric effects of pressure, mass flux, inlet subcooling on the transition boiling heat transfer are also investigated. From these comparisons, it can be seen that this model can identify the crucial mechanisms of forced convective transition boiling, and that the transition boiling heat fluxes including the maximum heat flux and the minimum film boiling heat flux are well predicted at low qualities/high pressures near 10 bar. In future, this model will be improved in the unstable film boiling stage and generalized for high quality and low pressure situations.

Transient pool boiling from downward-facing curved surfaces

Quenching experiments were performed to investigate the effects of wall thickness on pool boiling from downward-facing curved surfaces in water. Experiments employed three copper sections of the same diameter (50.8 mm) and surface radius (148 mm), but of different thicknesses (12.8, 20 and 30 mm). Local and average pool boiling curves were obtained at saturation and 5, 10 and 14 K subcooling. The maximum and minimum film boiling heat fluxes, which increased with increased subcooling, were independent of wall thickness > 19 mm and Biot number >0.8 and 0.008, respectively, indicating that boiling curves for the 20 and 30 mm thick sections were representative of quasi steady-state, but not those for the 12.8 mm thick section. When compared to that of a flat surface section of the same material and dimensions, the average pool boiling curve for the 12.8 mm thick section showed significant increases in the maximum heat flux (from 0.21 to 0.41 MW m −2) and the minimum film boiling heat flux (from 2 to 13 kW m −2) and about 11.5 and 60 K increase in the corresponding wall superheats, respectively.

Transient boiling caused by vapor film collapse at minimum heat flux in film boiling

This paper deals with the transient boiling heat transfer caused by vapor film collapse at minimum heat flux in film boiling for a horizontal rod heater in a pool of water under pressures from 100 to 400 kPa. Platinum rods of 2, 3, and 4 mm in diameter are used as the heater. In the transient boiling after the vapor film collapse, the heat flux increases at first from the minimum film boiling heat flux with rapid decrease in heater surface superheat. It arrives at a maximum heat flux value, and then decreases to nearly the same value as the minimum film boiling heat flux, where the heat transfer on the heater becomes again stationary in nucleate boiling or natural convection. The transient maximum heat flux is about 80% of the steady-state maximum heat flux of nucleate boiling independent of the pressure and heater diameter. The heater surface superheat at transient maximum heat flux is higher than the heater surface superheat at steady-state maximum heat flux. After the maximum heat flux is reached, the transient nucleate boiling heat flux approaches the steady-state nucleate boiling curve on heat-flux-versus-superheat graph and coincides with the curve in the lower heat flux region.

Improved pool boiling heat transfer to helium from treated surfaces and its application to superconducting magnets

Experiments reported here show that greatly increased heat transfer to a pool of boiling helium can be achieved by treating the surface being cooled. The essence of the treatment appears to lie in covering or partly covering the surface with a thin layer (~ 25 μm) of material of low thermal conductivity and low heat capacity (this feature is partly supported by theory): the superposition of rough particles (~ 100 μm) or porous material gives even greater improvement. The peak nucleate boiling heat flux is increased but the most marked effect is the increase in minimum film boiling heat flux by a factor of up to about 4 compared with a smooth copper surface. This phenomenon is useful in the construction of cryostatically stabilised magnets permitting the use of less copper stabilising material. An experiment is reported showing the increase in the recovery current in a typical conductor resulting from treating the surface.

Boiling Heat Transfer from Stabilized Superconducting Wire to Liquid Helium (II)

The boiling heat transfer from copper surface oriented vertically to liquid helium was investigated. The surface conditions had significant effects on the boiling heat transfer of liquid helium. The surface roughness, the surface history and the surface-fluid interface phenomena were discussed. Although the surface roughness showed a small effect in the nucleate boiling region, the peak flux did not change. The surfaces with various chemical films produced lower heat transfer coefficients in the nucleating boiling region, but they increased the minimum film boiling heat flux qmin. For the surface polished #600 emery paper, there was the much enormous hysteresis in the nucleate boiling heat transfer.

Film-Boiling Heat Transfer From a Horizontal Surface

Taylor-Helmholtz Hydrodynamic Instability and its significance with regard to film boiling heat transfer from a horizontal surface is discussed. It is shown that near the minimum film-boiling heat flux, the bubble spacing and growth rate is determined by Taylor Instability neglecting the effect of fluid depth and viscosity. Utilizing a simplified geometrical model, an analytical expression for the heat-transfer coefficient near the minimum in film pool boiling from a horizontal surface was derived. Combining this equation with the available correlation for the minimum heat flux yields an analytical equation for the temperature difference at the minimum, which defines the location of the minimum point. The above equations agree with the available experimental measurements made on n-pentane and carbon tetrachloride within ±10 per cent.