Counterintuitive Convection Flux Confined in a U-Shaped Tube

Heating a fluid causes a local expansion and the resulting buoyance of the rarefied fluid drives it upward. This phenomenon roughly defines heat convection. Convection is fairly well known due to our familiarity with a large number of convection systems in open air like candles, bonfires, barbecue grills, boiling pans, hot air balloons, etc. Most heat sources generate convection that can be seen or felt. Therefore, our common sense may trick us...