Purpose To modify a conventional evacuated tube, an improved asymmetric U-type evacuated tube (AUET) is proposed. This study aims to investigate the thermal and hydrodynamic performances of a modified tube and determine the optimal structural form. Design/methodology/approach Based on the variation of fluid proprieties with temperature, the formulated numerical model was validated and then deployed to investigate the natural circulation in the evacuated tubes. A dimensionless number was proposed to quantify the stratification effect. The influence of the degree of asymmetry of U-type evacuated tubes on the flow patterns, mass flow rate, temperature distribution, thermal stratification and energy conversion efficiency was studied. Findings When the degree of asymmetry is large, a higher velocity and better thermal stratification are achieved, thereby avoiding stagnant water at the bottom of the tubes simultaneously. Compared with the conventional evacuated tube, the improved evacuated tube exhibited a higher thermal efficiency. Originality/value The originally proposed AUET was proven to have better performance in avoiding stagnant water, reducing fluid mixing and improving the heat transfer efficiency.

Boundary conditions are usually the key problem in the establishment of a numerical model for simulation. An algorithmic method is needed to obtain a concrete numerical solution when the combined controlling equation sets are difficult to solve analytically. In this research, a type of algorithm known as the double forward method (DFM) is proposed to solve complex boundary conditions. The accuracy of the DFM is controllable, and it was found to be reliable when applying it to the water filling process in a water supply pipeline system. The DFM can also be used to solve multidimensional problems. In addition, the established water filling model in this study combined an open channel flow and a pressured flow, and a surge tank boundary condition was developed to fit the entire water filling process.

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Manhole cover is an important device of urban drainage infrastructures. The hydraulic characteristics of turbine-type manhole covers were studied through numerical simulations and physical experiments. The flow field characteristics and water surface were investigated. The drainage process is divided into two parts: free flow regime and submerged flow regime. Numerical and experimental results are in good agreement. It is indicated that the depth of water is constant in the later stage of unstable free drainage, while it changes with time and determines the discharge under the subsequent unstable submerged drainage condition. The influence of the depth on discharge is mainly reflected in the submerged drainage stage, in which period the discharge is linearly related to the square root of the depth. While in free flow regime, the discharge is affected by volume fraction of water with second order. The correlation between the depth and the discharge in the process of submerged flow is proposed based on dimensional harmony principle. With the characteristic of massive discharge, the design of turbine-type manhole cover provides one more choice in urban drainage construction.

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