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Thermal coefficient
Physical terms
Thermal coefficient is the volume expansion coefficient that measures the characteristics of uniform working medium.
Constant temperature compression coefficient
,
Adiabatic compressibility
And relative pressure coefficient.
- Chinese name
- Thermal coefficient
- Field of application
- physics
- Applied discipline
- physics
- unscramble
- Relative pressure coefficient
1.
Volume expansion
Coefficient (αv): the relative change rate of volume with temperature under constant pressure, that is, V, T and p represent volume, temperature and pressure respectively; The lower corner mark p indicates that the process taking place is carried out under constant pressure. For solids and liquids, αv changes only slightly with temperature and pressure, so when the temperature changes little, αv can be regarded as a constant; towards
Ideal gas
Alpha v=1/T.
2.
Constant temperature compression coefficient
(K) : the relative rate of change of the volume with pressure at a constant temperature, that is, the "-" sign indicates that the volume will shrink due to the increase of pressure. For solids and liquids, the K value changes little with temperature and pressure, so it can be regarded as a constant. right
Hubris
Think of a gas, K=1/p.
3.
Adiabatic compressibility
(KS) :
Adiabatic condition
The relative rate of change of the lower volume with pressure, that is, the middle and lower corner mark "s" indicates adiabatic. In general,KS≯K; Water at 4 ° C, KS=K.
Relative pressure coefficient (αp): the relative rate of change of pressure with temperature at a constant volume, that is, for an ideal gas, αp=1/T.
The relationship between each heat coefficient is:
The heat transfer coefficient was formerly called the total heat transfer coefficient. The current national standard specification is named as heat transfer coefficient.
Heat transfer coefficient K
Value means that under stable heat transfer conditions,
Enclosure structure
The temperature difference between the air on both sides is 1 degree (K or C), the heat transferred per unit time through the unit area, the unit is watts/(square meters · degrees) (W/ square meters ·K, where K can be replaced by ℃), reflecting the strength of the heat transfer process
It is not only related to the material, but also related to the specific process.
walling
Heat transfer coefficient K
It represents the heat transferred by the unit square meter wall area in unit time (W/(M2.K) when the temperature difference between the air on both sides of the wall (including all structural levels) is 1K(1℃) under stable heat transfer conditions, and the unit is W/(m2.k). That is, the heat transfer coefficient K includes all the structural levels of the wall and the air boundary layer on both sides. It characterizes the thermal performance of the wall insulation system, and studies have shown that
Exterior wall heat transfer coefficient
The reduction will significantly reduce building energy consumption.
For heat exchangers commonly used in air conditioning projects, if other attached heating resistance is not considered, the heat transfer coefficient K value of single-layer envelope structure can be calculated as follows:
K=1/(1/h1+δ/λ+1/h2) W/(㎡·°C)[2]
Among them, h1, h2 -- the heat exchange coefficient of the two surfaces of the envelope structure, W/(㎡·°C);
δ -- tube wall thickness, m;
λ - tube wall thermal conductivity, W/(m·°C).
Single layer structure thermal resistance R=δ/λ(m2.K/w)
Multilayer structure
Thermal resistance R=R1+R2+----Rn=δ1/λ1+δ2/λ2+----+δn/λn
Where: R1, R2, --Rn -- thermal resistance of each layer of material (m2.k/w); δ1, δ2, --δn -- material thickness of each layer (m); λ1, λ2, --λn -- thermal conductivity of each layer [W/(m.k)];
2, the envelope structure split convection heat transfer thermal resistance
Internal surface heat transfer resistance: Ri=1/h1;
Outer surface heat transfer resistance: Re=1/h2;
3, the heat transfer resistance of the enclosure structure
R0=Ri+R+Re
Where: Ri - internal surface heat transfer resistance (m2.K/W) (generally 0.11); Re - external surface heat transfer resistance (m2.K/W) (generally 0.04)
K=1/ R0(w/(m2.k))
Where: R0 - thermal resistance of envelope structure;
1. Calculation of the average heat transfer coefficient of the external wall under the influence of the surrounding thermal bridge
Km=(KpFp+Kb1Fb1+Kb2Fb2+ Kb3Fb3)/( Fp+ Fb1+Fb2+Fb3)
Where :Km - the average heat transfer coefficient of the external wall [W/ (m2.k)];
Kp - Heat transfer coefficient of the main part of the external wall [W/ (m2.k)];
Kb1, Kb2, Kb3 -- Heat transfer coefficients of the thermal bridge around the external wall [W/ (m2.k)];
Fp - the area of the main part of the exterior wall;
Fb1, Fb2, Fb3 - the area of the thermal bridge around the external wall;
Uw= (Af *Uf+Ag*Ug+Lg*Ψg)/(Af+Ag)
Formula:
Uw - Heat transfer coefficient of the whole window (W/m2·K);
Ug - Heat transfer coefficient of glass (W/m2·K);
Ag - Area of glass m2;
Uf - heat transfer coefficient of profiles (W/m2·K);
Af - area of the profile m2;
Lg - circumference of glass m;
Ψg - Linear heat transfer coefficient around the glass (W/m2·K), field test.
The window is composed of the frame material and the glass system. If the heat transfer of the glass and the frame fan is assumed to be strictly in parallel, the total thermal insulation coefficient R of the window is:
R =1/ [1/(Fg+Ff)*(Fg/Rg0+Ff/Rf0)] [3]
Where: the total thermal insulation coefficient corresponding to the RG0-glass system (refers to the thermal insulation coefficient of the inner air to the outer air of the object under consideration);
Rf0- The total thermal insulation coefficient corresponding to the frame fan (refers to the thermal insulation coefficient of the inside air to the outside air of the object under consideration);
Fg-glass area;
Ff- Area of window frame.
If the area ratio ηf is given:
Eta f = Ff / + Ff (Fg)
Expressed by the corresponding heat transfer coefficient U value, a simple and practical relationship is obtained:
U =Ug+ηf*(Uf-Ug)
The heat transfer coefficient of UG-glass;
Uf- Heat transfer coefficient of window frame.
At present, there are mainly the following methods for on-site detection of the thermal resistance heat transfer coefficient of the wall: heat flow meter method and power (known as the hot box method in the industry, in order to maintain unity and facilitate communication, the following is still called the hot box method), unsteady state method, respectively described below.
The basic idea of the heat flow meter method is to use the heat flow meter to measure the heat flow through the measured wall, while measuring the temperature on both sides of the wall, you can calculate the thermal resistance and heat transfer coefficient of the measured wall.
The probe of the heat flux meter is made according to the principle of thermoelectric effect and temperature gradient. There is a thermopile embedded in the probe, when a certain heat flow q flows vertically through the heat flow meter probe, there is a certain temperature difference △T on both sides of the substrate, this temperature difference makes the thermopile installed in the substrate produce a certain electromotive force. Due to the thickness of the substrate and the thermal conductivity is certain, under the condition of stable thermal conductivity, its heat flux is proportional to the temperature difference △T on both sides of the probe, and is also proportional to the generated electromotive force. The most fundamental requirement of the heat flux method is that the heat flow through the heat flux is both the heat flow through the measured object, and this heat flow is parallel to the direction of the temperature gradient, that is, the heat flow through the heat flux is a steady one-dimensional conduction, without considering the diffusion around. In this way, the cold end temperature and hot end temperature of the heat flow meter are measured at the same time, and the thermal resistance and heat transfer coefficient of the measured object can be calculated according to the formula
R=(T2-T1)/(E*C)
K=1/(Ri+R+Re)
Where K is the heat transfer coefficient [W/(m2.K)];
E is the heat flow meter reading (mv);
C is the coefficient of heat flow meter probe [W/(m2.mv)], the heat flow meter has been calibrated before delivery;
R is the thermal resistance of the measured object (m2.K/W);
T1 is the surface temperature of the cold end of the measured object (℃);
T2 is the surface temperature of the hot end of the measured object (℃);
Ri is the internal surface heat transfer resistance (m2.K/W);
Re is the heat transfer resistance of the outer surface (m2.K/W).
If limited by site conditions such as the use of shale particles
Waterproof coil
The roof is not smooth, and the external surface temperature cannot be accurately measured without treatment. Some use gypsum, fast cement, etc. to wipe out a smooth surface and then paste
Temperature sensor
Measurement of temperature, which in turn will introduce heating resistance, and the error caused by it cannot be accurately eliminated. In the internal and external surface temperature is not easy to determine, can use the shelter to measure the internal and external environmental temperature, several and through the heat flow meter heat flow, can be calculated according to the formula of the object to be measured heat transfer resistance and heat transfer coefficient:
R0=(Ta-Tb)/(E*C)
K=1/R0
Where is the heat transfer resistance of R0 measured object (m2.K/W);
The other symbols in the formula are the same as above.
The hot box method used for field detection is generally the protective hot box method. The measuring box is placed in a temperature-controlled space, and the internal temperature of the controlling measuring box is consistent with the indoor air temperature, so that there is no heat exchange between the measuring box and the external environment, and the other side is the outdoor natural conditions. Keep the temperature in the hot box higher than the outdoor temperature. In this way, the heat flow of the measured part is always transferred from the indoor to the outdoor, forming a one-dimensional heat flow. When the heat added in the hot box reaches a balance with the heat transferred through the measured part, the power of the metering box is the heat transferred in the measured part. Record the calorific value of the measuring box and the outdoor temperature in the hot box, and use formula 1 to calculate the heat transfer coefficient of the measured part can be obtained.
Some also take the double box method, that is, a protective box is set outside the measuring box, and the temperature of the measuring box and the protective box can be consistent during the test.
Operating conditions and methods:
- 1.
Select the test part according to the construction drawing of the building to be measured, should not be near the beam, plate, column and other hot bridge, the door and window of the room to be measured intact;
- 2.
The hot box is in close contact with the measured wall, and in order to achieve airtight, it is usually fixed on the back of the hot box with a pole;
- 3.
Attach a fixed temperature sensor. Fix the outdoor wall temperature sensor so that it is located in the center of the corresponding surface hot box, close to the wall surface, and shielded with tin foil to avoid direct sunlight. Fix the outdoor ambient temperature sensor so that it is located in the center of the corresponding surface hot box, away from the shadow of wall table 10 to 20 cm, and install a radiation shield to avoid direct sunlight. The indoor air temperature sensor is fixed so that it is located in the center of the measured room, 1.5 meters away from the wall, and the radiation shield is installed;
- 4.
Connect the temperature sensor and the hot box to the power temperature detector for measurement;
- 5.
The room where the hot box is placed is heated by electric heating, so that the temperature difference between the hot box and the room is less than 0.4℃, the temperature difference between the indoor and outdoor should be controlled above ℃, the temperature in the hot box is greater than the maximum outdoor temperature of 10℃ or more, if the average outdoor air temperature is above 8℃, the cold box should be used;
- 6.
The relative humidity of outdoor air must be below 60%, and the wind force must be less than level 3.
- 7.
It is advisable to carry out field testing after the external wall insulation construction is completed and the wall reaches the dry state.
- 8.
Use the computer to set the control temperature, the form of data collection, and set the recording interval and collection time. The acquisition instrument automatically records the power consumption of the hot box, the temperature in the hot box, the indoor temperature, the outdoor temperature, the internal and external surface temperature of the wall test site, the outdoor humidity and other parameters.
- 9.
The detection cycle is 72-96 hours, the temperature measurement range is -20-50℃, and all the data of not less than 24 hours after the stability of the cycle is collected.
After the measurement, the heat transfer coefficient is calculated automatically by the instrument, and it can also be manually used by EXCEL or Jinshan electronic watch
The heat transfer coefficient of the measured part is calculated by data processing.
Constant power plane heat source method is a more common method in unsteady state method, which is suitable for building materials and other
Thermal insulation material
Thermal physical test. The method of field detection is to artificially add a suitable plane constant heat source on the inner surface of the wall, heat the wall for a certain time, and identify the heat transfer coefficient of the wall by measuring the temperature response of the inner and outer surface of the wall. The heating part between the insulation cover plate and the wall is composed of 5 layers of materials. The heating plate C1, C2 and metal plate E1 and E2 are arranged symmetrically in two pieces, and the temperature on both sides of the insulation layer is controlled to be equal, so as to ensure that the heat emitted by the heating plate C1 flows to the wall. The effect of E1 plate on the wall surface is uniform heating. The wall internal surface temperature measuring thermocouple A and the wall external surface temperature measuring thermocouple D record the hourly temperature value.
- 1.
The system uses Artificial Neural Network (ANN) to simulate the solution process. It is divided into the following steps
- 2.
The heat transfer process of the wall designed by the system is an unsteady three-dimensional heat transfer process, which is affected by the action of the plane heat source inside the wall and the change of indoor and outdoor air temperature. The heat transfer program of the unsteady heat conduction wall is programmed accordingly. A solution model of wall heat transfer is established, a variety of boundary conditions and initial conditions are input, and the temperature field data of the wall after heating can be obtained by using the heat transfer program of the three-dimensional unsteady heat conduction wall.
- 3.
The obtained temperature field data, corresponding boundary conditions and initial conditions together form a sample set to train the network. In this study, since the wall temperature field data that can be measured by the experiment is only the temperature of the inner and outer surfaces of the wall, the following five parameters during the test time are taken as input samples of the neural network: average indoor temperature, average outdoor temperature, heat flux, and temperature of the inner and outer surfaces of the wall: the heat transfer coefficient of the wall is trained as output samples.
- 4.
The network reaches a stable state after a certain period of training, and the heat transfer coefficient of the wall can be mapped from the network by inputting each temperature value and heat flux value.
The heat transfer coefficient is a process quantity, and its size depends on the physical properties of the fluid on both sides of the wall, the flow rate, the shape of the solid surface, the thermal conductivity of the material and other factors. In the calculation of building heat loss, it is a parameter that characterizes the total heat transfer performance of the outer envelope, and its value depends on the materials used in the envelope, the structure and the environmental factors on both sides. The larger the heat transfer coefficient, the worse the insulation effect of the enclosure structure, such as the general single-layer 3mm thick glass metal window heat transfer coefficient of 6.4W/(mK), 370mm thick two sides plastered brick wall heat transfer coefficient of 1.59W/(mK).
[1]
The greater the K value, the more intense the heat transfer process. The heat transfer coefficient is not only mainly dependent on the physical properties of hot and cold fluids and their respective average flow velocity, but also related to many factors such as the thickness of the solid wall and the thermal conductivity of the material, which are generally determined by specific experiments and calculated by the heat transfer equation, or by calculating the total thermal resistance Rt per unit area of the heat transfer process.
