Keywords: CFD Simulation of Dew Humidity Ventilation
1. Introduction
Humidity environment problem is different from other indoor pollution problems. Too high or too low humidity will seriously affect the performance of buildings and the health of residents. If the humidity is too low, people will feel uncomfortable, such as dryness, which will cause cracks in the wall and deformation of the board. In addition, according to the research of Nordic scholars, the survival rate of influenza germs will increase significantly under low humidity. However, if the humidity is too high, on the one hand, it will cause condensation on the surface and inside of the wall, reduce the heat insulation and durability of the wall, and affect the building life [1]; On the other hand, when the humidity exceeds 70%, a large number of fungi will breed, causing allergic dermatitis, asthma and other diseases, affecting the health of residents [2-5]. China has a vast territory, and the indoor humidity environment in different regions presents different characteristics. Therefore, it is of great significance to study indoor humidity environment deeply.
With the rapid development of computer functions, CFD simulation technology is more and more used in the field of building environment, such as indoor temperature distribution, ventilation efficiency, micro-environment around human body and so on. But up to now, there is little research on indoor humidity distribution by CFD technology at home and abroad [6,7], and there is no research on the formation and development of indoor condensation.
2. CFD correction model considering humidity and condensation calculation
In this study, the standard k-ε turbulence model is adopted. However, considering that the air density will change when the water vapor content is high, which will affect the calculation of buoyancy, the model is modified according to the method proposed by Kondo et al. [8], and βx is introduced into the buoyancy term. Modified wet? See table 1 for the CFD calculation model of condensation. In addition, the calculation of condensation quantity is also considered in this study. Because the formation of condensation is a dynamic process, two indexes that change with time are put forward: the amount of condensation per unit wall area con (s, t) times time t,1); 2) Sum the dew on the wall, SUMCON(t). See figure 1 for the calculation method and its combination with CFD model.
In addition, the sources of moisture in buildings, such as toilets and kitchens, have large free surface area and the water temperature is generally higher than the ambient air temperature. The diffusion of water vapor molecules is accompanied by heat exchange. If this heat and moisture transfer is not considered at the same time, it will bring great errors to the indoor temperature and humidity distribution and airflow calculation results. Due to the lack of research in this field [9], we found through experiments that the heat and moisture transfer quantities M and qm of free water surface are determined by the following formulas, and then can be substituted into CFD calculation as internal boundary conditions in the form of upward flow of heat and moisture:
Figure 1 wet? Calculation flow of condensation in CFD condensation calculation model
Model table 1 calculation formula of continuous equation;
Equation of motion:
Transmission equation:
Transmission equation:
Heat transfer equation:
Water vapor transport equation:
Among them:
Vortex viscosity coefficient and other related items:
( 1)
(2)
Where-indoor air exchange times, h-1;
—saturated steam pressure corresponding to water temperature, kpa；;
-partial pressure of steam in ambient air, kpa；;
—— Water temperature,℃;
-Ambient air temperature,℃;
-latent heat of evaporation of water vapor, kj/kg;
-specific heat of air under constant pressure, kJ/(kgK).
-A newly proposed parameter, which represents the ratio of the actual heat dissipation caused by water vapor diffusion to the maximum heat dissipation during the total heat exchange of water vapor. In fact, because some water vapor only exchanged sensible heat on the way out of the water surface, there was no phase change, which should be a value between 0- 1. Using simple formula derivation and experimental fitting, the following formula can be arranged. Please refer to [10] for details.
(3)
3. Model test
Wet construction verification? In order to reveal the correctness of CFD calculation model, a model room was established in the artificial meteorological room of Tokyo Gas Company's technical research institute, and the model test was carried out, and the simulation results were compared with the experimental data.
An overview of this model is shown in Figure 2. The model cabin is made of polyethylene board, and various humidification conditions are adjusted and simulated in the cabin through the temperature control of ground humidifier and water. The humidification amount is obtained by measuring the weight change of humidifier with electronic balance. There is an opening on the outdoor wall of the small room, and a short ventilation pipe with a small axial fan can be connected to the opening. Different ventilation modes can be simulated by changing the position of the fan. The air volume in the small room is measured by a miniature anemometer installed in the short ventilation pipe. In addition to the outdoor temperature and humidity, 14 temperature and humidity measuring element (THP-B4, Shen Rong Company) is arranged in the central part of the room to measure the temperature and humidity distribution.
See Table 2 for test and simulation conditions. Working condition 1 is the steady calculation of indoor temperature and humidity distribution, and working condition 2 is the dynamic calculation of condensation formation and development. Ventilation methods are all mechanical ventilation methods.
Test conditions table 2
Working condition number
Meteorological room condition
Humidifier water temperature
(℃)
humidity
(gram/hour)
Ventilation capacity
(m3/hour)
inlet temperature
Temperature (℃) and humidity (%)114.347.044.627.517.014.3219.445.068+06538+
Figure 2 Summary of model test and arrangement of measuring points
4. Comparison of test and calculation results
4. 1 Verification of temperature and humidity distribution
Fig. 3 shows the comparison between the test of working condition 1 and CFD calculation, in which the test data is the result when the measured values of all measuring points reach a steady state. In the simulation value, qm=486W/m2 is the actual water vapor diffusion heat dissipation calculated by Formula 2) and Formula 3) (at this time, Fm is about 0.52). For comparison, we also assume that all steam evaporates in gas phase, and there is no phase change. Diffusion and heat dissipation are all composed of sensible heat exchange (qm= 184W/m2). Steam evaporates in total heat exchange (q'= 1026W/m2). As can be seen from the figure, the diffusion and heat dissipation of water vapor have great influence on the indoor temperature distribution. For example, when qm= 1026W/m2 is adopted, the temperature near the ceiling is about 2 degrees higher than the measured value. Some studies [1 1] advocate that the total heat exchange capacity should be used to estimate the heat dissipation of water vapor diffusion when calculating the heat and humidity load in the bathroom, which will inevitably lead to great calculation errors. In contrast, because the indoor humidity is mainly determined by the mass balance of water vapor, the diffusion heat dissipation value has little effect on the average level of indoor humidity, but because of the different buoyancy effect of heat flux, the air flow mode near the water surface leads to subtle changes in the humidity distribution. Generally speaking, the calculated results of q'=486W/m2 are most consistent with the measured data, regardless of temperature and humidity.
Fig. 4 shows the CFD simulation results of the flow field and temperature and humidity field on the measurement section. As can be seen from the figure, the hot and humid plume formed by the water surface on this section is hardly affected by the ventilation of the small room, and the temperature and humidity stratification is very obvious.
Fig. 3 Comparison of measured and simulated temperature and humidity distribution (working condition 1)
Fig. 4 CFD calculation results (left: airflow field; Medium: temperature field; Right: humidity field)
4.2 Verification of Dew Formation and Development Process
Fig. 5 shows a comparison between the test results of case 2 and the CFD calculation results. The test and simulation time is about 30 minutes. In order to better form surface condensation and prevent indoor air humidity from reaching 100%, a preheating device was used to heat the intake air to 25.2 degrees in this test. As can be seen from the figure, both the actual measurement and simulation show that the humidity of the measuring points (P 1-7 and P2-7) on the ceiling reaches saturation after about 20 minutes, indicating that dew condensation has occurred. This shows that although there is no good method to directly measure the dew condensation problem, by comparing the dynamic distribution of temperature and humidity in a small room, the variation law of calculated value and simulated value with time is basically the same, so it can be considered that it is feasible to analyze the dew condensation problem by using this calculation model.
For the second case, we use wet? The dynamic simulation of dew condensation is carried out by CFD calculation model, and the simulation time is 1 hour. Fig. 6 shows the dew distribution at four time points. About 20 minutes after the start of the experiment, condensation first appeared in the upper corner behind the cabin, and then developed along the ceiling and side walls at a faster speed. After 45 minutes, the condensation area is basically stable, but the condensation volume continues to increase. Judging from the amount of condensation, there are more condensation on the two side walls and ceiling at the back of the chamber. This is because the front side wall and the ground are close to the air inlet, and the hot air raises the temperature of these walls.
Fig. 5 Comparison of dynamic changes of measured and simulated temperature and humidity (working condition 2)
Variation diagram of condensation distribution with time simulated by CFD.
5. Influence of ventilation on condensation
Ventilation is one of the important means to solve the problem of dew condensation, but so far there is little quantitative analysis in this regard. We use CFD to dehumidify? The condensation calculation model discusses the influence of different ventilation volume and ventilation mode on condensation through three examples.
Example 1: i.e. case 2;
Example 2: The ventilation rate is increased from 7.9 to 9.4m3/h, and other conditions remain unchanged;
Example 3: the ventilation mode is changed to the mechanical ventilation mode of upper delivery and lower discharge, and other conditions remain unchanged.
Fig. 7 shows the simulation results of condensation distribution in Examples 2 and 3 (t = 60 minutes). Compared with fig. 6, due to the increase of ventilation, the dewing area of Example 2 is correspondingly reduced, especially at the lower part of the side wall. The ventilation rate of Example 3 is exactly the same as that of Example 1, but the dew distribution shape is completely different. Due to the adoption of the way of upward delivery and downward discharge, the phenomenon of water vapor rising caused by buoyancy near the water surface is suppressed, and the condensation on the ceiling and the upper part of the side wall is reduced. Fig. 8 shows the calculation results of each wall and the total amount of condensation in three examples (t = 60 minutes). As can be seen from the figure, the total exposure of Example 2 and Example 3 is only 26% and 20% of that of Example 1 respectively.
Fig. 7 Influence of different ventilation volume and ventilation mode on condensation distribution (left: Example 2; Right: Example 3) Figure 8 Dew condensation changes under different ventilation rates and modes.
6. Conclusion
In order to study the indoor humidity distribution and dew condensation by CFD technology, a wet-type? Condensation CFD calculation model, through model verification and example calculation, the following conclusions can be drawn:
1) Whether the condensation is not considered, only the steady calculation of humidity distribution is considered, or the unsteady calculation of condensation is considered, the calculation results of CFD model are in good agreement with the experimental results. This shows that the model can be used for detailed analysis of indoor humidity field and condensation. Especially for the problem of dew condensation, the application of CFD provides a very important research means when there is no effective measurement method at present.
2) The problem of heat and moisture transfer at the same time is also involved in this study, and a new calculation method is given and applied to CFD calculation.
3) Even if the air volume is the same, different ventilation modes have great influence on indoor humidity distribution and overall humidity level. Under the condition of high indoor humidity, the problems of dehumidification and condensation can be solved more effectively through the optimal design of ventilation mode.
7. Reference
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