Fenghao Wang

Flow Patterns of Cross-Flow around a Varicose Cylinder

The near wake of a varicose cylinder has been experimentally investigated using Laser Induced Fluorescence (LIF) and Digital Particle Imaging Velocimetry (DPIV). The work aims to provide understanding to the mechanism of the cross flow around varicose cylinder as well as to comprehend why the introduction of relatively small degrees of spanwise waviness can have a significant effect on drag reduction and suppression of the cylinder vibration. The evolution of the flow patterns and the corresponding vortex interactions are obtained. The experimental results indicated that the wake width and the formation length vary along the span of the varicose cylinder. A wider wake and a longer formation length were observed in the saddle plane. In addition, an interpretation of the three-dimensional wake structures is postulated and conceptually shown. The numerical simulation by 3-D finite volume method is successful in predicting the flow features found by the experiments.

Experimental investigation on dynamic performance of air-source heat pump water heater using R134a

This paper describes the experiment carried out to analyse the effects of improper refrigerant charge and incorrect thermal expansion valve (TXV) opening on the ASHPWH using R134a. The performance of the system were measured in combinations of four refrigerant charges of 70%, 85%, 100% and 115% of the optimal value and three TXV opening of 30%, 45% and 60% of full opening at ambient temperature of 20°C D/15°C W (dry-bulb temperature/wet-bulb temperature). The results showed that the maximum coefficient of performances (COPs) were nearly 33%, 31.9% and 4.4% higher than that of 70%, 85% and 115% of the optimal refrigerant charge at 60% TXV opening, respectively. And it was found that the increase in TXV opening can enhance the COP when water temperature exceeded 30°C. Besides, an exergy analysis was conducted to evaluate the thermodynamic performance of the system and components in this study.

Comparison of indoor air distribution and thermal environment for different combinations of radiant heating systems with mechanical ventilation systems

A hybrid system with a radiant heating system and a mechanical ventilation system, which is regarded as an advanced heating, ventilation and air-conditioning (HVAC) system, has been applied in many modern buildings worldwide. To date, almost no studies focused on comparative analysis of the indoor air distribution and the thermal environment for all combinations of radiant heating systems with mechanical ventilation systems. Therefore, in this article, the indoor air distribution and the thermal environment were comparatively analyzed in a room with floor heating (FH) or ceiling heating (CH) and mixing ventilation (MV) or displacement ventilation (DV) when the supply air temperature ranged from 15.0℃ to 19.0℃. The results showed that the temperature effectiveness values were 1.05–1.16 and 0.95–1.02 for MVu2009+u2009FH and MVu2009+u2009CH, respectively, and they were 0.78–0.91 and 0.51–0.67 for DVu2009+u2009FH and DVu2009+u2009CH, respectively. The Predicted Mean Vote values were from 0.24 to 0.45 and from 0.11 to 0.43 for MVu2009+u2009FH and MVu2009+u2009CH, respectively, and from 0.01 to 0.23 and from −0.41 to 0.10 for DVu2009+u2009FH and DVu2009+u2009CH, respectively. Hence, MVu2009+u2009FH had the largest temperature effectiveness and Predicted Mean Vote, and DVu2009+u2009CH had the smallest values. In addition, the vertical air temperature differences for MVu2009+u2009FH and MVu2009+u2009CH were all within the comfort zone according to ISO 7730, but exceeded the comfort zone for DVu2009+u2009FH and DVu2009+u2009CH when the supply air temperature was less than 17℃ and 19℃, respectively. The air distribution effectiveness values for MVu2009+u2009FH and MVu2009+u2009CH were close to the recommended value for MV in the ASHRAE Standard 62.1, and those for DVu2009+u2009FH and DVu2009+u2009CH were slightly less than the recommended value for displacement ventilation. The results in this article are relevant and useful in the process of selection and design of a hybrid system with a radiant heating system and a mechanical ventilation system in practice. Practical application: The supply air temperature is one of key parameters for the design and operation of a hybrid system with a radiant heating system and a mechanical ventilation system. The results in this article may contribute to the design and operation of a hybrid system when taking in account the indoor air quality and thermal comfort.

Effects of wall masonry layer’s thermophysical properties and insulation position on time lag and decrement factor

In this study, effects of the thermophysical properties of the wall masonry layer on the time lag and decrement factor of the composite wall were investigated numerically. For this purpose, the time lag and the decrement factor of six kinds of composite walls were calculated using the software FLUENT. The results show that the time lag would increase with an increase in the heat capacity of the masonry wall layer and would decrease with an increase in the thermal conductivity of the masonry wall; the decrement factor would decrease with an increase in the heat capacity of the masonry wall layer, but would increase with an increase in the thermal conductivity of the masonry wall. In the walls with one insulation layer, the wall with insulation on the outer surface would always have the smallest decrement factor. In the walls with two insulation layers, the wall with the insulation partly in the middle and partly on the outer surface would always have the highest time lag and the wall with the insulation partly on the inner surface and partly on the outer surface would always have the smallest decrement factor.

Simplified number of transfer unit formulas for the thermal performance calculation of multi-pass fin-tube heat exchangers

The known effectiveness–number of transfer units relationships of multi-pass fin-tube heat exchangers consist of a large number of terms in the series and are not expressed directly in terms of the number of transfer units; as a result, it is difficult to calculate the number of transfer units and the heat transfer coefficient of heat exchangers for a given effectiveness and thermal capacity ratio. To solve this problem, heat transfer models for multi-pass fin-tube heat exchangers based on the heat balance method were established in this article, and simplified number of transfer units formulas for thermal performance calculation were derived according to the flow arrangements of heat exchangers and the flow directions of two fluids. When the number of flow passes in a heat exchanger is greater than four, the relative errors between the calculated number of transfer units by these proposed formulas and those calculated by the effectiveness–number of transfer units relationships of the counter-flow or parallel-flow heat exchangers were all less than 2%. When the number of flow passes in a fin-tube heat exchanger is less than four, a two-pass counter-flow fin-tube heat exchanger was selected as the validation case due to the similar flow characteristics of heat exchangers. The results indicated that the relative errors between the calculated heat transfer coefficients of the heat exchanger according to these proposed number of transfer units formulas and those calculated using the LMTD factor were all less than 5% when the inlet water temperature was in the range of 45°C to 60°C and the water flow rate was in the range of 50 to 170 kg/h. These proposed number of transfer units formulas were determined to be valid and could be beneficial for the design of multi-pass fin-tube heat exchangers.