Research on the Ratio between the Convection and Radiation Heat Exchange Amount of the Face and Head of Manikin in the Thermal Comfort Condition

In this paper, the radiation heat exchange coefficient of the exposed parts of human body is firstly obtained through pre-experiment, which is used to separate the total output heat exchange amount of the manikin. Then the heat exchange amount of manikin under different airflow organizations of personal ventilation, mixed ventilation and seat ventilation are respectively studied. The value of convection heat exchange and radiation heat exchange of the face and head of human body are compared. And variable conditions are studied for different air temperatures and volumes. It is concluded that when the supply air temperature changes between 22℃ and 30°C with human body thermally comfortable, the total heat exchange of the face and head changes from 38 W/m 2 to 137 W/m 2 , and the range of the radiation heat exchange amount is 26 W/m 2 to 67 W/m 2 . The convection heat exchange amount changes between 6 W/m 2 and 110 W/m 2 , and the ratio of radiation heat exchange to convection heat exchange changes from 0.5 to 6.7.


1.Introduction
The heat balance between the human body and the surrounding environment is the basis to obtain comfort in steady-state environment.The heat exchange between the body and the environment is dominated by convection and radiation.And the ratio of the heat exchange may affect the choice of the design of airflow.However, there is a lack of relevant studies on the ratio of convection and radiation heat exchange between human and environment on the condition of human thermal comfort.In this paper, taking the manikin in thermal comfort as a study object, are carried out an experimental research on the range and proportion of convection and radiation heat exchange in naked body parts of the face and head.
The current research on the impact of heat exchange on the thermal comfort of the human body is mainly about the cooling problem of radiant panels.Dongliang Zhang et al. found the laws of proportions of the net radiation heat exchange, convection heat exchange and total heat exchange of the system based on the measured indoor parameters [1] .Yuying Liang et al. found that the decoupled radiation cooling device with low cooling temperature has the potential to increase the cooling capacity by directly utilizing the low-temperature cooling source, which can significantly reduce the area of the radiation cooling panel while maintaining the thermal comfort of the human body [2] .Lin et al. found that the temperature fluctuation and the change of vertical temperature distribution are small when adopting the mixed ventilation and radiation air-conditioning system through experiment and questionnaires [3] .The above scholars have only studied the comfort or heat of the radiant air conditioning system.But there is a gap in the demand for the ratio of radiation and convection heat exchange under the condition of human thermal comfort.
In the current research on thermal comfort of manikin, Zhang et al. established the Berkeley thermal comfort model and divided the body into 19 parts for the first time.They also proposed the thermal sensory model of each part [4] .Mengying Zhang et al. established a human hand model to study the effects of wind velocity and direction on heat exchange and airflow.They found that the radiation heat loss increased slightly with the increasing wind velocity [5] .Jingxian Xu et al. found that the air distribution of the room has a great influence on the convection heat exchange of the human body and the emissivity of the wall has a great influence on the radiation heat exchange [6] .Meng Kong et al. found that the heat loss of the manikin was positively correlated with the velocity, but negatively correlated with the air temperature [7] .The literatures have used the manikin to study the amount of heat exchange in parts of body under the thermal comfort, but they have not obtained the requirements for the convection and radiation heat exchange of each part.
In this paper, the radiation heat exchange coefficient of the exposed parts is obtained through preliminary experiments first, which is used to separate the total heat exchange of the manikin.Then the variable conditions of different temperatures and airflow organizations are studied.Finally, the proportions of convection and radiation heat exchange under different conditions are researched.The influence of the proportion of radiation and convection heat exchange on thermal comfort provides the basis for the choice of air distribution.

Method and methodology 2.1 Introduction of the experimental manikin
The experimental manikin consists of 20 parts.The area of face is 0.026 m 2 and the area of head is 0.077 m 2 .The laboratory is on the second basement floor, which can completely disregard the influence of solar radiation and other external thermal disturbances.And the size of the experimental room where the manikin is located is 3.6 m×2.7 m×2.7 m (length ×width ×height).In order to ensure the same radiation rate of the environment to the manikin, the walls of the experimental room are covered by tin foil.The thermocouple is aimed to measure the wall temperature.There are 3 thermocouples evenly arranged on the left and right walls of the manikin, 2 thermocouples on the front and back walls, 3 thermocouples on the ceiling and 2 thermocouples on the ground.The size of each wall and the layout of thermocouples are shown in Figure 1.

Variable working conditions of manikin in thermal comfort state
In this experiment, the manikin is controlled in the condition of thermal comfort.That is, the manikin and the environment come to heat equilibrium.The manikin is connected to the computer through the USB (Universal Serial Bus), which can directly read the skin temperature of 20 parts of the manikin with an accuracy of ±0.01℃ and its total heat exchange with an accuracy of ±0.1 W/m 2 .There are three types of airflow organization in this experiment: personal ventilation, diffuser mixed ventilation (hereinafter referred to as mixed ventilation) and seat ventilation, as shown in Figure 2. The distance between the personalized nozzle and the face is 50cm and the diameter of the throat of the diffuser is 0.2m.The air supply temperature are conducted in 22℃, 26℃, and 30℃ and the volume are set to 100m 3 /h, 130m 3 /h and 160m 3 /h respectively.The specific working condition are shown in Table1.In the experiment, the skin temperature and total heat exchange output by the manikin are mainly automatic recorded.The data is collected under the conditions when the environment temperature, wall temperature and skin temperature are all stable.

3.Separation mechanism of total heat exchange amount of manikin
Although the computer can directly read the skin temperature and total heat exchange amount of the manikin, it cannot directly obtain the value of its radiation heat exchange amount and convection heat exchange amount.Thus, separating the total heat exchange amount into convection part and radiation part is the key to this research.In the traditional calculation of convection heat exchange, an anemometer is used to measure the velocity around the manikin to obtain the surface heat exchange coefficient.But the anemometer will destroy the flow field around the manikin and cause the inaccuracy.Therefore, a new method for obtaining radiation heat exchange amount from the perspective of radiation heat exchange principle is proposed for the first time and the brief description is as follows: (1) According to the principle of radiation heat exchange, the radiation heat exchange coefficients of exposed parts of face and head are determined through preliminary experiments.According to Stefan-Boltzmann law of radiation, formula (1) [8] : Where Qr is the radiation heat exchange amount between the surface of the human body and environment, W/m 2 .εp is the average emissivity of the surface of body.fcl is the area coefficient of clothing.feff is the effective radiation area coefficient of the human body.σ is Boltzmann constant, σ=5.67×10 -8 W/(m 2 •K 4 ).Tcl is the average temperature of the surface of the human body, K. Tr is the average radiation temperature of the environment, K. β0 is the average radiation heat exchange coefficient of parts of the human body, W/(m 2 •K 4 ).
The radiation heat exchange coefficient β0 involved in the calculation is the average radiation heat exchange coefficient of face and head obtained under laboratory conditions.εp, fcl, feff, and σ are all constants in the specific manikin and the experimental environment.
(2) In the preliminary experiment, the hood with the same volume as the face and head is used to supply air.The air supply temperature is controlled to be equal to the skin temperature.At this time, the temperature difference between the manikin and environment causes that the convection heat exchange is zero.Thus, the total heat exchange amount of the manikin is equal to the radiation heat exchange, then the radiation heat exchange coefficient β0 between the manikin and environment which can also be used in other conditions is calculated.
(3)In the process of solving β0, it is necessary to obtain the radiation temperature corresponding to each part from equation(2) [9] and the angle factor of each part to walls of the room in equation ( 2) is solved by the S2S surface radiation model in Fluent.
Where Ti is the wall temperature, K. Fi is the angle factor of the body part to the wall, i=1, 2, ..., 6.
The average radiation heat exchange coefficients of the face and head are calculated as 0.996 W/(m 2 •K 4 ) and 0.822 W/(m 2 •K 4 ) respectively.
(4)The radiation heat exchange amount of exposed parts can be obtained by the radiation heat exchange temperature and the average radiation heat exchange coefficient.Then the convection heat exchange can be calculated by subtracting the radiation heat exchange from the known total heat exchange.
In this way, the total heat exchange output of the manikin can be separated into radiation heat exchange and convection heat exchange.

4.Variation characteristics of convection and radiation heat exchange amount in typical exposed parts
The ratio α of radiation to convection heat exchange amount between the typical exposed parts of the manikin and the environment is defined in formula (3).
Where Qr is the radiation heat exchange of the exposed parts, W/m 2 .Qc is the convection heat exchange amount, W/m 2 .
Figure 3 shows the characteristics of skin temperature, average radiant temperature, the convection and radiation heat exchange amount of the face and head with different air temperature.Figure 4 shows the characteristics of heat exchange with different airflow organizations.The heat exchange range of the face and head can be obtained based on 27 conditions , as shown in Table 2.It can be seen from Figures and Table 2 that under the different airflow organizations, since the face and head are affected by the outlet, the decline of the supplying air temperature will cause the skin temperature decreasing correspondingly.Furthermore, the skin's adaptability reduces the skin temperature from 34°C to 31°C, which is less than the temperature drop of 8°C in supplying air temperature.Since the average radiation temperature is mainly affected by the air temperature, the decrease of the air temperature will lead to a decline in the average radiation temperature.However, the range of the average radiation temperature is 22℃~28℃, which is also less than the 8℃ drop of the supplying air temperature.At the same time, as to the different airflow organizations, although both the skin temperature and the average radiation temperature reduce with the decrease of the air temperature, the amplitude of decrease is different.The average radiation temperature range of personal ventilation is between 22°C and 27°C, smaller than that of seat ventilation and mixed ventilation.
Under the three airflow organizations with the same air volume, the difference between the skin temperature and the radiation temperature increases, which will gradually increase the radiation heat exchange amount of the face and head.Meanwhile, the difference between the air temperature and the skin temperature will gradually increase the convection heat exchange amount.When the air volume is same, the convection heat exchange of the face and head is the largest under the personal ventilation which can reach 93 W/m 2 .The velocity range of the face of personal ventilation is 2.0-2.3m/s,greater than that of two airflow organizations.The velocity of the face and head of seat ventilation is below 1.5m/s.The convection heat exchange is the smallest under the seat ventilation.Because the outlet of the seat ventilation is far from the face and head with low velocity, the convection heat exchange is minimal which reach 6 W/m 2 .
The ratio α of radiation to convection heat exchange amount in the face is between 0.47 and 3.37, and the ratio of head is between 0.82 and 6.65.Furthermore, the ratio of heat exchange will decline with the decrease of the air temperature.The ratio α of the face and head is the largest in seat ventilation, followed by the mixed ventilation, and the personal ventilation is the smallest.Except for the personal ventilation, the heat exchange ratio α of the face and head under other conditions is greater than 1.It means that the convection heat exchange amount accounts for the main part of the total heat exchange in the case of personal ventilation.But the radiation heat exchange accounts for the main part of the total heat exchange under other working conditions .

5.Conclusion
In general, the following conclusions are obtained: 1)The heat exchange of the manikin of the face and head all decrease with the increase of the air temperature, while the skin temperature and the average radiation temperature increase with the increase of air temperature.The range of the radiant heat exchange amount of the face and head is about 26W/m 2 ~60 W/m 2. .The ratio of radiation to convection heat exchange increases with the increase of air temperature, which means that the higher the air temperature, the greater the proportion of radiation heat exchange.The range of the ratio of radiation to convection heat exchange of face is 0.47 to 3.37 and the range of the ratio of heat exchange of head is 0.82 to 6.65.
2)When the air temperature changes between 22°C and 30°C with the manikin in thermal comfort, the total heat exchange range of the face and head is 40 to 137 W/m 2 , and the range of convection heat exchange is 6 to 93 W/m 2 .The range of radiation heat exchange is 26 to 60 W/m 2 , while the ratio of radiation to convection heat exchange changes from 0.47 to 6.65.
3)The convection heat exchange accounts for the main part of the total heat exchange in the case of personal ventilation But the radiation heat exchange accounts for the main part of the total heat exchange under other working conditions .
(a)Personal ventilation (b)Seat ventilation Fig.2.Schematic diagram of airflow organization

3 .Fig. 4 .
Fig.4.Convection and radiation heat exchange amount of face and head with airflow organization The research was supported by National Natural Science Foundation of China (51878408) and Research and Development Center of Transport Industry of Health& Epidemic Prevention Technology by Ministry of Transport, PRC.

Table 1 .
The condition of the manikin in thermal comfort

Table 2 .
Heat exchange range of typical exposed parts(W/m²)