The existing single-blow, maximum-slope transient technique has been commonly applied for determining heat transfer performance of heat exchanger units or surfaces. However, results lack in accuracy and technical difficulties arise when it is applied to heat exchanger surfaces or units with NTU value smaller than 3.5. In the present paper, a modified single-blow technique is developed which can be applied to systems of any NTU values. Both theoretical analysis and experiments are performed. The validity of the present technique is borne out by the good agreement between experimental data and the existing theoretical solutions.
The dynamic characteristics of a gas bubble situated in a viscoelastic fluid of three-parameter Oldroyd model is analytically investigated. Consideration is given to the influence of the thermodynamic process of the gas inside the bubble. An equation is derived which governs the timewise variation of the bubble size. Numerical results are obtained for the collapse of a bubble due to a sudden increase in the system pressure. The influence of the physical parameters on the collapse is investigated. It is disclosed that, in the presence of elasticity, the viscous damping effects on the bubble collapse are less in viscoelastic liquids than in pure viscous fluids. The dynamic behavior of a gas bubble in a three-parameter viscoelastic fluid is simulated by a mechanical system. The mechanism of the bubble behavior may be satisfactorily explained using the model.
Interfacial flow structures in small liquid drops evaporating on flat plates are cinematographically investigated using the methods of direct photography and laser shadowgraphy. Various liquids of relatively low boiling point were evaporated on glass and copper plates at room temperature. The laser shadowgraph records the flow patterns simultaneously at both the liquid-air interface and the liquid-solid interface, from which the evaporation rate is determined. It reveals the existence of three distinct flow structures at the liquid air interface: stable, substable, and unstable. An interfacial flow map is constructed. The direct photography is employed to study the morphology during the entire process of the unstable-interface type evaporation. The mechanism of ripple formation which enhances the evaporation rate is found to be caused by hydrophilicity of the liquid.
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