What effect does choking a vertical thermosiphon have on the heat transfer rate?

What effect does choking a vertical thermosiphon have on the heat transfer rate?
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Answer :

Choking down on the channel outlet nozzle and piping reduces the circulation rate through a heat  exchanger. Since the tubeside heat-transfer rate depends on velocity, the heat transfer is lower at  reduced recirculation rates. A rule of thumb says that the inside flow area of the channel outlet nozzle  and piping should be the same as the flow area inside the tubing. Shell Oil in an experimental study  showed that a ratio of 0.7 in nozzle flow area/tube flow area reduced the heat flux by 10%. A ratio of 0.4  cut the heat flux almost in half.
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