Recommendations for the heat transfer assessment for natural convection boiling of microfinned surfaces

The paper presents the generalized correlations for the heat transfer coefficients for natural convection boiling of microfinned surfaces based on the experimental measurements.


Introduction
The microfinned surface elements on the flat horizontal plate produced by the deforming cutting method is one of the most effective ways of heat transfer intensification for natural convection boiling [1,2]. The deforming cutting method is a simple and no waste technology.It was revealed that for the two-and the three-dimensional microstructured surfaces for the element height of 90-750 µm, element thickness of 25-320 µm, and longitudinal gap width of 40-500 µm the heat transfer coefficients is growing up to a factor of 9, and the critical heat flux magnitudes are rising up to a factorof 4.1. The maximal values of the heat transfer intensification were observed for the three-dimensional surface elements. The results of studying the heat transfer enhancement for these surfaces for the boiling of the distilled water, 98%ethanol, and 60% water solution of the glycerin are presented in [3,4].

Research results
In the studied range of the heat flux q=410 3 ÷210 6 w/m 2 were observed the convective, surface, and bubble boiling regimes. the experiments were carried out for the boiling of a saturated fluid. Before the running the experiments the degasification of the working fluid and the artificial grinding of the studied surfaces was made. During the process of the grinding for the studied elements, theworking fluid was boiled for several times. The experimental for each type of the surface elements were duplicated and the duration of the experimental investigation for one type of the surface element was 2-3 days.
During the analysis was assumed the average value of the n=0.6. It is deferrers little from the adopted values [5,7] of the influence of a heat flux on the heat transfer coefficient for the boiling for the dependence Nu~Kq0,7. This effect was observed in [5] for the boiling on the microporous surface. transfer coefficient with the increase of the vaporization velocity Kq. The second regime the rate of the heat flux is equal to the smooth surface the third regime is observed the effect when the liquid is pushing aside from the microfinned surfaces. This effect is analogous to the film boiling. The heat transfer coefficient for the third regime is equal or becomes less than the magnitudes for the heat transfer coefficient of boiling for the smooth surface.
Generally, for all of three regimes explained in [5] the power n of the Kq could be different for the boiling on the microfinned and microporous surfaces. For instance, this effect could be observed for the samples №22 and №33 (Fig. 1). The change of the wall roughness is leading to the considerable increase of the heat transfer coefficient, because the amount of evaporations centers is defined by the wall roughness [5,7]. As the efficient evaporation centers could be used the dimples and the grooves on the surface where the liquid and the gas could be hold. This assumption allows to evaluate the influence of the dimensionless geometrical parameters of the fins on the relative heat transfer coefficient, including the relative transverse gap of the two-dimensional micro-fins /l0. It should me mentioned that the value of the transversal gap was ranged for   m and was considerably lover then the value of the capillarity constant l0=2,5  m. Due to this fact, the capillarity effect was observed on the prospected samples. It may be noted, that for the large values of the two-dimensional structures (>l0) the gaps are filled by the fluid and do not participate in the boiling as the active evaporation centers.
The studied values of the ratio /l0allows us to quantify the microfinned channels as the capillary type [8].
The experimental values of the average heat transfer coefficients for the natural convection boiling of distilled water on the two-dimensional microfinned surfaces at the atmospheric pressure could be generalized by the equation (1) The deviation of the experimental data from the equation above do not exceed30 % with the confidence probability of 95% (Fig. 3).   Table 1.
The experimental data for the average heat transfer coefficients for natural convection boiling ( Figure 6) of distilled water on the microfinned surfaces with the three-dimensional tubular elements (3D) for the atmospheric pressure could be generalized by the equation (Figure 7):  10  31  15  34  17  35  20  37  21  38  28  39  29 40 +45% Figure 6. Experimental results for heat transfer rate for boiling of distilled water on the surfaces with three-dimensional tubular fins (3D). Notation is shown in Table 1. Line -Micheev equation. Figure 7. Dependence of the relative heat transfer rate for boiling of distilled water on the surfaces with three-dimentional tubular fins (3D) versus criterion Kq. Notation is shown in Table 1.

Conclusion
In the paper are given the recommendations for increasing the heat transfer for boiling on microfinned surfaces and shown the general correlations for heat transfer coefficient for natural convection boiling on surfaces with two-and three-dimensional microfinned surfaces from the operational Kq and dimensionless geometrical criteria KГ.