I. Convection = Conduction + Flow Transport
Heat transfer between a fluid and a wall is called convective heat transfer. It is essentially the conduction of the fluid near the wall, then carried away by flow. Newton's law of cooling simplifies it to:
Q = h · A · ΔT
h is the surface heat transfer coefficient (W/m²·K), A is the area, and ΔT is the temperature difference between the wall and the fluid. Among the three heat transfer modes, convection is the hardest to calculate and the most worth optimizing, because h is greatly affected by flow velocity, physical properties, and channel shape.
II. Forced Convection vs. Natural Convection
- Natural convection: Driven by density difference (hot rises, cold sinks), h is typically 5~25 W/m²·K, very weak;
- Forced convection: Driven by fans and pumps, h can reach 50~200 or even higher, the main force of industrial heat exchange.
For the same heat exchanger, doubling the air volume raises h to about 1.5~2 times (approx. h ∝ u^0.8), and the heat transfer rises accordingly. This is also why, in drying exhaust waste heat recovery, the make-up air side wind speed design is often more critical than imagined.
III. Why Turbulence Is Stronger Than Laminar Flow
The faster the fluid, the easier it enters turbulence. In turbulent flow, the fluid mixes violently, the "boundary layer" near the wall that is almost stationary thins, thermal resistance drops sharply, and h rises significantly. The cost is increased power consumption of fans/pumps and pressure drop. Design is about finding the balance between the two.
IV. Two Practical Suggestions for Selection
- Do not fear increasing flow velocity: Within the allowable pressure drop range, moderate speed-up is the most cost-effective means of efficiency improvement;
- Pay attention to whether both sides match: A common pitfall is one side with very high wind speed and the other very low, so the overall h is dragged down by the "short board" side; the flow velocities of both sides should be matched as much as possible.
The efficiency of a heat exchanger often does not depend on how expensive the plates are, but on whether you dare to push the fluid fast and chaotic enough.