Influencing factors of countercurrent heat transfer
1. The cause of flow According to the different causes of flow, convective heat transfer can be divided into two categories: natural convection heat transfer and forced convection heat transfer.
Natural convection is the flow caused by the floating force generated by the non-uniform temperature field inside the fluid causing the density field to be uneven.
Forced convection refers to the flow of fluid under the action of a fan, water pump or other external force. Generally speaking, the natural convection flow rate is lower than the forced flow, so the surface heat transfer coefficient is small.
2. Flow regime The fluid flow regime has laminar, transitional, and turbulent flows.
When the fluid flows in a laminar flow, the fluid flows in layers, without intermixing between the layers, and the heat transfer perpendicular to the flow direction is mainly by heat conduction (ie, molecular diffusion).
In the turbulent flow, there is disturbance and mixing in the fluid, and the convection effect is strengthened.
In addition to the heat conduction mode, the heat transfer mainly depends on the macroscopic turbulent pulsation of the fluid, so the surface heat transfer coefficient of the turbulent convective heat transfer is higher than that of the laminar convective heat transfer. Big
3. Physical properties of fluids The physical properties of convective heat transfer are mainly: thermal conductivity, density, specific heat capacity, and degree of refraction.
4. Fluid 10,000-phase phase change Fluid is sometimes accompanied by phase changes during convective heat transfer. For example, in a refrigeration system, the refrigerant liquid becomes vaporized by the heat of the vaporization bath as it flows through the evaporator.
When the refrigerant vapor flows through the condenser, the heat of the condensation bath is turned into a liquid state, and the heat exchange when the fluid undergoes a phase change is stronger than the heat exchange when there is no phase change.
5. Geometrical factors of the heat transfer surface Geometrical factors such as the geometry, size, relative position and surface leg length of the heat transfer surface affect the fluid heat transfer of the fluid.