The subject of the present work is the modelling of the liquid streamwise flow velocity in the two-phase boundary layer in subcooled boiling flow under the influence of the vapor bubbles. Subcooled boiling flow experiments were carried out in a horizontal test channel in order to investigate the interaction between the bubbles and the liquid phase. The heater surface was located at the bottom of the test channel. The near-wall liquid flow velocity was measured using a two-component laser-Doppler anemometer. Based on the experimental data a model is proposed to describe the impact of the gaseous phase on the motion of the liquid in the subcooled boiling regime. It was observed that the axial velocity profiles near the wall follow a logarithmic law similar to that used in turbulent single-phase flow over rough surfaces. Based on this finding it is suggested to model the influence of the bubbles on the liquid flow analogously to the effect of a surface roughness. The correlation developed for an equivalent surface roughness associated with the bubbles yields good agreement of the modeled axial velocity profiles with the experimental data.
The requirement for the highest possible heat transfer rates in compact, efficient cooling systems can often only be met by providing for a transition to subcooled boiling flow in strongly heated wall regions. The significantly higher heat transfer rates achievable with boiling can help keep the temperatures of the structure on an acceptable level. It has been shown in many experimental studies that special surface finish or porous coatings on the heated surfaces can intensify the nucleate boiling process markedly. Most of those experiments were carried out with water or refrigerants. The present work investigates the potential of this method to enhance the subcooled boiling heat transfer in automotive cooling systems using a mixture of ethylene-glycol and de-ionized water as the coolant. Subcooled boiling flow experiments were carried out in a vertical test channel considering two different types of coated surfaces and one uncoated surface as a reference. The experimental results of the present work clearly demonstrate that the concept of enhancing boiling by modifying the microstructure of the heated surface can be successfully applied to automotive cooling systems. The observed increase in the heat transfer rates differ markedly for the two considered porous coatings, though. Based on the experimental data, a heat transfer model for subcooled boiling flow using a power-additive superposition approach is proposed. The model assumes the total wall heat flux as a nonlinear combination of a convective and a nucleate boiling contribution, both obtained from well-established semiempirical correlations. The wall heat fluxes predicted by the proposed model are in very good agreement with the experimental data for all considered flow conditions and surface types.
The spray cooling technology is a key technology in the continuous casting process, which has a significant influence on the product quality. In order to ensure high quality standards at different operating conditions, modern continuous casting machines provide the possibility to vary the cooling with respect to intensity, width, stand off distances, etc. For design purposes, numerical simulation is used to calculate the feasible operating range of the caster. During casting operation, an online thermal tracking model, e.g., Dynacs, computes the set points of the cooling intensity depending on the actual casting conditions. Both tools use mathematical models which require precise information about the cooling characteristics of spray nozzles. The subject of the present work is the modeling of the convective heat transfer due to spray cooling using flat spray air-mist nozzles, which are widely used in slab casting machines. Measurements of the spray pattern and air/water flows were carried out on a spray test stand in order to investigate the profiles of the water flux at different air/water pressures and different distances from the nozzle tip to the surface. The air-to-water ratio was chosen according to the well-proven Siemens VAI air pressure control curve. The heat transfer coefficients due to spray cooling were measured using an experimental stand which allows spray cooling experiments up to a surface temperature of 1250 C. Based on the obtained experimental data a heat transfer correlation for air-mist sprays is developed to describe the effect of the different operating conditions on the spray cooling heat transfer coefficient. The correlation shows good agreement between the heat transfer coefficient profiles from prediction and experiment.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.