Iztok Golobič

Effects of heater-side factors on the saturated pool boiling critical heat flux

Abstract There are many aspects of critical heat flux (CHF) that still require clarification. Particularly important is the role that the heater-side factors play in CHF. The saturated pool boiling CHF on horizontally suspended, vertically oriented, ribbon heaters in FC-72 fluid was measured. On the basis of comprehensive experimental results (50 data sets) and data sets available in the literature, the effects of ribbon thermal properties and thickness were determined. A practical criterion is proposed for sizing noninsulated ribbon heaters without significant conductance/capacitance effects.

Surface effects on pool boiling CHF

The influence of the surface of horizontally and vertically oriented ribbon heaters made of stainless steel 302 or steel 1010 on pool boiling CHF in FC-72 or H 2 O was studied. The ribbon was 50 mm long, 5 mm high and 76.4 μm thick. Surface treatment was performed with different sandpapers and etching in diluted acid. In the centre line average R a range from 0.02 to 1.5 μm, the pool boiling CHF on stainless steel 302 ribbon heaters in FC-72 increased from 87.6 to 115.4 kW/m 2 and in H 2 O it increased from 410.8 to 499.7 kW/m 2 , while for steel 1010 ribbon heaters the CHF increased in H 2 O from 309.7 to 443.9 kW/m 2 On an etched heater surface, pool boiling CHF in H 2 O on both stainless steel 302 and steel 1010 increased by 51% with respect to a sanded surface with the same R a . It appears that a multiparametric heating surface characteristic and the surface chemistry parameters need to be included in the study.

Measurement of certain parameters influencing activity of nucleation sites in pool boiling

The influence of decreasing the distance between artificially activated nucleation sites and a naturally activated nucleation site on their activity was studied. A naturally activated nucleation site was activated during saturated water pool boiling at atmospheric pressure using electrical or laser heating of a relatively large surface of 25 μm thick copper or titanium foil. The interaction between nucleation sites located at various distances was observed during their simultaneous activation with a laser heated spot, which could be moved along the surface. The results of measurements indicate that the influence of active nucleation sites on each others activity are mutual. Moving a laser heated spot along the surface was used to show the influence of location on the activity of nucleation sites.

Corresponding states correlation for maximum heat flux in two-phase closed thermosyphon

Abstract A method is presented for the prediction of performance limit in two-phase closed thermosyphon based on the thermodynamical corresponding states principle. Molecular weight, critical temperature, critical pressure and acentric factor are needed instead of thermodynamic and transport properties of fluid at a given temperature. Developed corresponding states correlation for maximum heat flux is found to be in fairly good agreement with the 436 experimental data for 12 nonpolar and polar fluids with fill ratio greater than 0.4 in vertical two-phase closed thermosyphons with an inner diameter of 3–34 mm and the ratio of the evaporator length to its diameter with values of up to 325.

Interactions between laser-activated nucleation sites in pool boiling

Abstract A technique of activating nucleation sites by heating them with laser beams has been developed to enable direct measurements of interactions between active nucleation sites. Interactions between two, three and four simultaneously active nucleation sites in various arrangements on 25 μm thick copper and titanium foil in saturated water pool boiling were studied experimentally. The experiments were performed with heated spot diameters ranging from 1.66 to 5.23 mm and heat fluxes up to 560 kW/m2. The interaction between two laser activated nucleation sites occurs as a net decrease of activity of both sites. It is possible that the activity of both decreases simultaneously, or the activity of one increases, while the activity of the other nucleation site decreases simultaneously. Similar behaviour was also observed in the experiments involving three and four active nucleation sites. In the extreme case, one nucleation site can deactivate the other. Local surface characteristics can play an important role in the interaction between the nucleation sites.

Comparison of a Mechanistic Model for Nucleate Boiling with Experimental Spatio-Temporal Data

Mechanistic numerical simulations have been developed for pool nucleate boiling involving large groups of nucleation sites that are non-uniformly distributed spatially and have different activation superheats. The simulations model the temperature field in the heated wall accurately and use approximations for events in the liquid–vapour space. This paper describes the first attempt to compare the numerical simulations with spatio-temporal experimental data at a similar level of detail. The experimental data were obtained during pool boiling of water at atmospheric pressure on a horizontal, electrically heated stainless steel plate 0.13 mm thick. They consist of wall temperature fields measured on the back of the plate by liquid crystal thermography at a sampling rate of 200 Hz over a period of 30 s. Methods of image analysis have been developed to deduce the time, position, nucleation superheat and size of the cooled area for every bubble nucleation event during this period. The paper discusses the methodology of using some of the experimental data as input for the simulations and the remainder for validation. Because of the high-dimensional dynamics and possibly chaotic nature of nucleate boiling, the validation must be based on statistical properties over a large area and a long period. This preliminary study is restricted to a single heat flux.

SOFTWARE SENSOR FOR BIOMASS CONCENTRATION MONITORING DURING INDUSTRIAL FERMENTATION

For the on-line monitoring of the parameters and states of aerobic fermentation batch processes three parallel software sensors for biomass concentration monitoring were developed, which use mutually independent measured input signals: the oxygen uptake rate, the carbon dioxide excretion rate and the reaction heat flow. The software sensors are based on a linear model, where the parameters of the model are estimated using the recursive least squares method. The model was upgraded with a mechanism for a time-varying forgetting factor to enable good tracking of the estimated parameters during rapid and large changes in the non-linear and, in certain phases of the production, very dynamic process. The estimates given by all three software sensors are in good agreement with the measurements acquired off-line. The developed software sensors make it possible to achieve accurate process identification, mutual control of individual sensors and fault detection in the process. In this way, better supervision and more accurate monitoring of the industrial fermentation process is possible, where reliability of the operation is of key importance.

Pool boiling CHF on a laser heated thin plate

Abstract The technique of laser heating a thin metal foil was used to experimentally observe local phenomena in pool boiling CHF on a small flat surface. The experiments were performed with saturated water boiling on 25 μm thick copper and titanium foils with heating surface diameters ranging from 2.27 to 9 mm. The number of simultaneously active nucleation sites depends on the heat flux and the size and material of the laser heated surface. A reduction in the heating surface diameter DH moves the transition between one and several simultaneously active nucleation sites towards higher heat fluxes. CHF for small heating surfaces is lower than that for entirely heated surfaces. The variation of the ratio of bubble/vapour mass diameter to the heating surface diameter, DB/DH, with DH is similar to the variation of CHF with DH. CHF is significantly influenced by the number of simultaneously active nucleation sites. The influence of heating of the surrounding surface on decreasing DH, which was observed using two laser beams with different diameters was more intense at higher DH on Cu foil than on Ti foil.

Scalable surface microstructuring by a fiber laser for controlled nucleate boiling performance of high- and low-surface-tension fluids

Nucleate boiling enables effective cooling and heat transfer at low temperature differences between a heated surface and the surrounding fluid. It is utilized in many applications, ranging from large power plants to small microelectronics. To enhance the boiling process by minimization of the surface temperature and increase the maximum attainable heat flux, several approaches for surface modifications were recently developed. However, each of them has at least one important drawback, including challenging and expensive production, mechanical and/or thermal instability or problematic scale-up. Herein, a straightforward, robust and flexible method using a nanosecond fiber laser for production of surfaces with multi-scale micro-cavities (with diameters ranging from 0.2 to 10 μm) is developed. Examination of these surfaces in two very contrasting fluids - water, which is polar, has high surface tension and high latent heat of vaporization; and non-polar, dielectric tetradecafluorohexane (FC-72) with low surface tension and much lower latent heat - confirms that such surfaces enable enhanced heat transfer and controlled boiling in combination with diverse fluids. This demonstration suggests that the developed method has the potential to overcome the current limitations for further miniaturization of microelectronic devices and to increase performance and safety in high heat flux systems.

Effect of Nucleation Cavities on Enhanced Boiling Heat Transfer in Microchannels

ABSTRACT Boiling instabilities, high temperatures of the onset of boiling (ONB), and early transition to dryout are some of the insufficiently resolved issues of flow boiling in microchannels. This article addresses the flow boiling challenges with the incorporation of flow restrictors to reduce the boiling instabilities and hinder vapor backflows. In addition, the temperature of the ONB was lowered and the heat transfer coefficient was increased during boiling with the fabrication of potential nucleation cavities in the microchannel walls and bottom. Experiments were conducted with degassed double-distilled water in arrays of microchannels with the hydraulic diameter ranging from 25 to 80 µm, whereas the nucleation cavities characteristic sizes varied from 2 to 12 µm. The temperatures of the ONB were up to 35 K lower in the microchannel array with properly sized nucleation cavities compared to arrays of microchannels, in which the etched nucleation cavities were less suitable. The combined effect of fabricated nucleation cavities and interconnected microchannels increased the heat transfer coefficient from three to 10 times depending on the size of the etched nucleation cavities and the transferred heat flux in the microchannel arrays.