Water is heated by submerging 50-mm diameter, thin-walled copper tubes in a tank and passing hot combustion gases (Tg = 750 K) through the tubes. To enhance heat transfer to the water, four straight fins of uniform cross section, which form a cross, are inserted in each tube. The fins are 5 mm thick and are also made of copper (k = 400 W/m · K).
If the tube surface temperature is Ts = 350 K and the gas-side convection coefficient is hg = 30 W/m2 · K, what is the rate of heat transfer to the water per meter of pipe length?