Xiaozhou Wu

Air distribution in a multi-occupant room with mixing or displacement ventilation with or without floor or ceiling heating

This study performed a comparative analysis of the air distribution in a multi-occupant room with mixing or displacement ventilation and the effect of adding floor or ceiling heating to each of them. The vertical distribution of indoor air temperature and velocity in the occupied zone and the horizontal distribution of indoor containment concentration in the breathing zone were measured for all six systems with a supply air temperature of 19.0°C and an air change rate of 4.2 h−1. The results showed that the mean vertical air temperature difference in the occupied zone varied from 0.1°C to 0.6°C; the mean local turbulence intensity varied from 12.0% to 14.1% with mixing ventilation with or without floor or ceiling heating, and the corresponding values were 1.5°C to 2.5°C and 7.3% to 9.8% with displacement ventilation with or without floor or ceiling heating. Mean air distribution effectiveness varied from 0.93 to 1.0 for mixing ventilation and from 1.06 to 1.14 for displacement ventilation with or without floor or ceiling heating. The results are relevant to the design and control of mixing and displacement ventilation with or without floor or ceiling heating in a multi-occupant room.

Air Distribution and Ventilation Effectiveness in a Room with Floor/Ceiling Heating and Mixing/Displacement Ventilation

The present study investigated different combinations of floor/ceiling heating with mixing/displacement ventilation and their impacts on the indoor air distribution and ventilation effectiveness. Measurements were performed in a room during heating season in December. The results show that indoor vertical air temperature differences and air velocities for different hybrid systems are less than 3 °C and 0.2 m/s when supply air temperature is 19 °C, air change rate is 4.2 h−1, and heated surface temperature of floor/ceiling heating system is 25 °C. Ventilation effectiveness of mixing ventilation system combined with floor/ceiling heating systems is approximately equal to 1.0, and ventilation effectiveness of displacement ventilation system combined with floor/ceiling heating systems ranges from 1.0 to 1.2. The floor/ceiling heating systems combined with mixing ventilation system have more uniform indoor air distribution but smaller ventilation effectiveness compared with the floor/ceiling heating systems combined with displacement ventilation system. With regard to the building heat loss increased by non-uniform indoor air distribution and small ventilation effectiveness, there should be an optimal combination of floor/ceiling heating with mixing/displacement ventilation to have the minimal building heat loss.

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 MV + FH and MV + CH, respectively, and they were 0.78–0.91 and 0.51–0.67 for DV + FH and DV + CH, respectively. The Predicted Mean Vote values were from 0.24 to 0.45 and from 0.11 to 0.43 for MV + FH and MV + CH, respectively, and from 0.01 to 0.23 and from −0.41 to 0.10 for DV + FH and DV + CH, respectively. Hence, MV + FH had the largest temperature effectiveness and Predicted Mean Vote, and DV + CH had the smallest values. In addition, the vertical air temperature differences for MV + FH and MV + CH were all within the comfort zone according to ISO 7730, but exceeded the comfort zone for DV + FH and DV + CH when the supply air temperature was less than 17℃ and 19℃, respectively. The air distribution effectiveness values for MV + FH and MV + CH were close to the recommended value for MV in the ASHRAE Standard 62.1, and those for DV + FH and DV + CH 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.

Experimental Study on Heat Dissipation Performance of Forced Convector with U Shaped Fin-Tube

Forced convector, which is one of the low temperature heating end users, not only high efficient but also conveniently adjustable, fits for low temperature metering heating system which can raise energy efficiency. So in this paper, in order to convenient for engineering application of forced convector, heat dissipation calculation method and heat dissipation characteristic equation are put forward. Heat dissipation performances of forced convector were studied with the two parameters of heat transfer coefficients and mean temperature differences when inlet water temperature is from 45 to 60 and water flow rate is from 50 kg/h to 170 kg/h. Experimental results show that: the heat transfer coefficients are almost independent of the mean temperature difference under constant water flow rate, but have significant exponential relationship with variable water flow rate. The final calculation results of heat dissipation characteristic equation of forced convector are closed to experimental data.

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.