scholarly journals Numerical investigation of film thickness variation on falling film tubular heat exchangers at different Reynolds number

2019 ◽  
Vol 1240 ◽  
pp. 012013
Author(s):  
Awdhesh Kumar Poddar ◽  
Nirmal Kant Singh
Keyword(s):  
Reynolds Number ◽  
Film Thickness ◽  
Numerical Investigation ◽  
Heat Exchangers ◽  
Thickness Variation ◽  
Falling Film

NUMERICAL INVESTIGATION OF FILM DISTRIBUTION IN HORIZONTAL-TUBE FALLING FILM EVAPORATOR

2011 ◽  
Vol 19 (03) ◽  
pp. 177-183 ◽  
Author(s):  
JIN-BO CHEN ◽  
QING-GANG QIU
Keyword(s):  
Numerical Simulation ◽  
Fluid Flow ◽  
Film Thickness ◽  
Liquid Film ◽  
Numerical Investigation ◽  
Horizontal Tube ◽  
Flow Density ◽  
Falling Film ◽  
Film Distribution ◽  
Simulation Results

The technique of horizontal-tube falling film has been used in the cooling and heating industries such as refrigeration systems, heating systems and ocean thermal energy conversion systems. The comprehensive performance of evaporator is directly affected by the film distribution characteristics outside tubes. In this paper, numerical investigation was performed to predict the film characteristics outside the tubes in horizontal-tube falling film evaporator. The effects of liquid flow rate, tube diameter and the circular degree of tube on the film thickness were presented. The numerical simulation results were compared with that of the empirical equations for calculating the falling film thickness, and agreements between them were reasonable. Numerical simulation results show that, at the fixed fluid flow density, the liquid film is thicker on the upper and lower tube and the thinnest liquid film appears at angle of about 120°. The results also indicate that, when the fluid flow density decreases to a certain value, the local dryout spot on the surface of the tube would occur. In addition, the film thickness decreases with the increases of the tube diameter at the fixed fluid flow density.

Theoretical Analysis on Film Thickness of Intertube Falling-Film Flow With a Countercurrent Gas Flow

2011 ◽  
Author(s):  
Binglu Ruan ◽  
Huan Li ◽  
Qiuwang Wang
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Reynolds Number ◽  
Shear Stress ◽  
Film Thickness ◽  
Gas Flow ◽  
Film Flow ◽  
Falling Film ◽  
Two Phase ◽  
Flow Effects

In falling–film type of heat exchangers, gas/vapor usually exists, and its effect on falling-film mode transitions and heat transfer could not be neglected. It could impact the film thickness, which is an important parameter to determine the thin-film heat transfer performance, or even destroy falling-film modes and significantly deteriorate the heat transfer. However, there have been very few studies of countercurrent gas flow effects on the film thickness. In this paper, the falling-film film thickness with and without liquid-gas interfacial shear stress due to the countercurrent gas flow was studied. A two-phase empirical correlation is used to solve the momentum equation. Calculation results were compared with available experimental data in literatures for validation. Reasonable agreement was achieved. Thus, the two-phase correlation for predicting shear stress of a thin film flow inside a vertical rectangular channel has been extended to a new type of flow. Effects of film Reynolds number, gas velocity, and gas-channel equivalent hydraulic diameter on the film thickness were studied. It is shown that the countercurrent gas flow thickened the falling film. The increased film thickness can shift the mode transitional Reynolds number and reduce the heat transfer coefficient, corroborating the conjecture in our earlier work.

scholarly journals Net-exergetic, hydraulic and thermal optimization of coaxial heat exchangers using fixed flow conditions instead of fixed flow rates

2021 ◽  
Vol 9 (1) ◽  
Author(s):  
Tobias Blanke ◽  
Markus Hagenkamp ◽  
Bernd Döring ◽  
Joachim Göttsche ◽  
Vitali Reger ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Laminar Flow ◽  
Mass Flow ◽  
Flow Rate ◽  
Heat Exchangers ◽  
Circular Ring ◽  
Flow Conditions ◽  
Flow Rates ◽  
Mass Flow Rates ◽  
Pipe Radius

AbstractPrevious studies optimized the dimensions of coaxial heat exchangers using constant mass flow rates as a boundary condition. They show a thermal optimal circular ring width of nearly zero. Hydraulically optimal is an inner to outer pipe radius ratio of 0.65 for turbulent and 0.68 for laminar flow types. In contrast, in this study, flow conditions in the circular ring are kept constant (a set of fixed Reynolds numbers) during optimization. This approach ensures fixed flow conditions and prevents inappropriately high or low mass flow rates. The optimization is carried out for three objectives: Maximum energy gain, minimum hydraulic effort and eventually optimum net-exergy balance. The optimization changes the inner pipe radius and mass flow rate but not the Reynolds number of the circular ring. The thermal calculations base on Hellström’s borehole resistance and the hydraulic optimization on individually calculated linear loss of head coefficients. Increasing the inner pipe radius results in decreased hydraulic losses in the inner pipe but increased losses in the circular ring. The net-exergy difference is a key performance indicator and combines thermal and hydraulic calculations. It is the difference between thermal exergy flux and hydraulic effort. The Reynolds number in the circular ring is instead of the mass flow rate constant during all optimizations. The result from a thermal perspective is an optimal width of the circular ring of nearly zero. The hydraulically optimal inner pipe radius is 54% of the outer pipe radius for laminar flow and 60% for turbulent flow scenarios. Net-exergetic optimization shows a predominant influence of hydraulic losses, especially for small temperature gains. The exact result depends on the earth’s thermal properties and the flow type. Conclusively, coaxial geothermal probes’ design should focus on the hydraulic optimum and take the thermal optimum as a secondary criterion due to the dominating hydraulics.

Sign in / Sign up

Export Citation Format

Share Document