Q = h*A*(tw-t∞)=q*A
Among them:
Q is the heat exchanged between the solid surface per unit area and the fluid per unit time, which is called heat flux density, and the unit is w/m 2;
Tw and t∞ are the temperatures of solid surface and fluid, respectively, in k;
A is the wall area of m 2; ;
Q is the heat transfer on the area a, and the unit is w;
H is called the surface convection heat transfer coefficient, and the unit is w/(m 2.k).
Approximate order of convective heat transfer coefficient (unit: w/(m2 * k));
Natural convection of air 5 ~ 25
Forced convection of gas 20 ~ 100
Natural convection of water 200 ~ 1000
Forced convection of water 1000 ~ 15000
Forced convection of oil 50 ~ 1500
Water vapor condensation 5000 ~ 15000
500 ~ 2000 for organic vapor condensation.
The boiling point of water is 2500 ~ 25000.
The heat transfer process caused by the macroscopic movement of particles in various parts of the fluid will only occur when there is fluid flow. The unit is W/(㎡*K), which indicates the convective heat transfer rate and reflects the convective heat transfer rate. The greater the convective heat transfer coefficient, the faster the convective heat transfer. Convective heat transfer coefficient is divided into local total heat transfer coefficient and total heat transfer coefficient, and the total heat transfer coefficient is always close to the fluid with small α.
Extended data:
The sign of surface heat transfer coefficient is H, (α); Q =h(Ts-Tr), where: Ts is the surface temperature; Tr is the reference temperature to characterize the external environment. Heat. SI unit: w/(m2 k) (Watts per square meter). ?
Newton's cooling formula: q=h(Tw-Tf) when the fluid is heated.
When the fluid is cooled, Q=h(Tf-Tw).
Where Tw and Tf are wall temperature and fluid temperature, respectively,℃. If the temperature difference (also called temperature and pressure) is recorded as Δ t and always positive, Newton's cooling formula can be expressed as: q = h Δ t.
φ= haδT
Where q is the heat flow in watts per square meter (w/m2) and φ is the heat flow in watts (w).
There are four ways to get the expression of surface heat transfer coefficient h:
1, the analytical method, mathematically describes the partial differential equation of a certain convection heat transfer problem and the corresponding solution method of the definite solution conditions, thus obtaining the analytical solutions of the velocity field and the temperature field.
2. The experimental method under the guidance of similarity principle is the main way to obtain the surface heat transfer coefficient.
3. Analogy method, by studying momentum transfer and heat transfer or similar characteristics, to establish the relationship between surface heat transfer coefficient and resistance coefficient.
4. Numerical method: On the basis of solving the thermal conductivity, the dispersion of convection term and numerical treatment of pressure gradient term in momentum equation are added to obtain the surface heat transfer coefficient.
References:
Baidu encyclopedia-surface heat transfer coefficient