Tube-to-tube heat transfer degradation effect on finned-tube heat exchangers

Tube-to-tube heat transfer is suspected to affect the overall capacity of most finned-tube heat exchangers running subcooled (single-phase) simple substances, or zeotropic mixtures, as coolants. A simplified numerical model is presented and used for simulating the heat transfer process of air flowing in between two parallel fins of a finned-tube heat exchanger with known tube temperatures. The objective of the numerical simulation is to detect the existence of tube-to-tube heat transfer and the corresponding detrimental impact it might have on the overall capacity of the heat exchanger. The numerical simulation considers a 4 x 3 finned-tube, single-phase, air-water heat exchanger with data provided by the National Institute of Standards and Technology (NIST). Results indicate the existence of strong tube-to-tube heat transfer along the fins (particularly among neighboring tubes presenting very different temperatures, as expected). The tube-to-tube heat transfer phenomenon is shown to account for approximately 20% of the heat exchanger capacity. Thin adiabatic layers placed along the fin surface are shown, also through numerical simulations, to be very effective in interrupting the tube-to-tube heat transfer phenomenon. A simplified analytical procedure to estimate the tube-to-tube heat transfer effect of any finned-tube heat exchanger, suggested as a design tool, is satisfactorily tested against the numerical results.