Prediction of the Critical Heat Flux in Subcooled Flow Boiling in Round Tube Using an Improved Mechanistic Model

2010 ◽  
Author(s):  
Na Yu ◽  
Yu Zhang
Keyword(s):  
Model Comparison ◽  
Flow Boiling ◽  
Mechanistic Model ◽  
Force Balance ◽  
Experimental Result ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
Proposed Model ◽  
Crisis Phenomenon ◽  
Good Agreement

Among the existing CHF models, the bubble crowding model and the liquid sublayer dryout model have been well accepted for subcooled flow boiling. But both of the two models couldn’t give explanation about some details in the boiling crisis phenomenon according to photographic result. The aim of the present paper is to provide an improved synthesized model containing the characteristic of the above two models and then to give a comprehensive explanation about CHF. In the present model, the conservation equations of mass and energy are solved to derive the CHF formula. The length and velocity of the vapor blanket and the thickness of the liquid sublayer are needed. The quality and void fraction in bubble region and the core region are calculated by a homogeneous assumption. The vapor blanket length is thought to be equal to the Helmholtz wavelength and it is obtained from several parameters in the bubble region. The velocity of the vapor blanket is connected to the flow velocity of the bubble layer. The thickness of the sublayer is determined by a force balance on the vapor blanket, which is also related to the condition of the bubble region. About 1100 experimental points have been selected to verify the proposed model. Comparison between the predictions by the proposed model and the experimental result shows a good agreement that more than 90% of these data are predicted within ±20%.

A Control Volume Approach for Investigating Forces on a Departing Bubble Under Subcooled Flow Boiling

1995 ◽  
Vol 117 (4) ◽  
pp. 990-997 ◽  
Author(s):  
S. G. Kandlikar ◽  
B. J. Stumm
Keyword(s):  
Surface Tension ◽  
Flow Boiling ◽  
Rectangular Channel ◽  
Small Diameter ◽  
Contact Angles ◽  
Force Balance ◽  
Subcooled Flow Boiling ◽  
New Model ◽  
Subcooled Flow ◽  
Static Conditions

This paper presents the theoretical and experimental work conducted on nucleating vapor bubbles in subcooled flow boiling of water at atmospheric pressure and 60°C in a 3 mm × 50 mm × 400 mm long rectangular channel. A new model is developed for analyzing forces acting on the vapor bubble under pseudo-static conditions corresponding to the thermally controlled region of bubble growth. The model considers two separate control volumes for the front and rear regions of the bubble. The forces due to surface tension, buoyancy, drag, pressure difference, and momentum changes are considered, and the effects of different upstream and downstream contact angles are included. These angles and the departure bubble diameters are measured from the top and the side views of bubbles recorded on a video camera through a microscope. The new model and the experimental study confirm that the bubble removal in flow boiling for small diameter bubbles investigated in this study (less than 500 μm) is initiated at the front edge of the bubble through a sweep-removal mechanism. Previous models available in the literature consider a force balance on the entire bubble, and are therefore unable to address the effect of a significant reduction in the component of the surface tension force in the flow direction at the leading edge caused by an increase in the upstream contact angle.

Wall Heat Flux Partitioning During Subcooled Flow Boiling: Part II—Model Validation

2005 ◽  
Vol 127 (2) ◽  
pp. 141-148 ◽  
Author(s):  
Nilanjana Basu ◽  
Gopinath R. Warrier ◽  
Vijay K. Dhir
Keyword(s):  
Experimental Data ◽  
Heat Flux ◽  
Flow Boiling ◽  
Mechanistic Model ◽  
Wall Heat Flux ◽  
Experimental Conditions ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
Flux Partitioning ◽  
Model Predictions

A mechanistic model for wall heat flux partitioning during subcooled flow boiling proposed in Part I of this two-part paper, is validated in this part. As the first step of the validation process, the developed model was applied to experimental data obtained as part of this study. Comparison of the model predictions with the present data shows good agreement. In order to further validate/exercise the model, it was then applied to several data sets available in the literature. Though the data in the literature were for experimental conditions vastly different from those from which the model was originally developed, reasonable agreement between the model predictions and the experimental data were observed. This indicates that the proposed model can be extended to other flow conditions provided the submodels cover the conditions of the experiments. Future work should be directed towards improvement of the various submodels involved to extend their range of applicability, especially the ones related to bubble dynamics. Additionally, it must be kept in mind that the model as proposed is strictly only applicable to vertical up-flow and may not be applicable to other orientations.

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