What happens to the flow rate at the nozzle when fuel temperature decreases?

When the temperature of the fuel decreases, the viscosity of the fuel oil increases. This increase in viscosity means that the fuel flows more slowly through the nozzle due to its thicker consistency. However, the pressure at which the fuel is delivered to the nozzle, often maintained by the pump, does not significantly change. As a result, although the resistance to flow increases, the nozzle's design is meant to accommodate and maintain a specific flow rate under given conditions.

For most oil burning systems, as fuel temperatures drop, the system compensates in various ways, including using nozzle designs that create more atomization and ensuring a consistent spray pattern. However, the flow rate from the nozzle is primarily determined by the pressure differential across the nozzle and the temperature-related viscosity changes. Therefore, a decrease in temperature generally leads to a flow rate that does not increase, contrary to the answer you provided. Thus, the correct understanding is that the flow rate actually decreases as viscosity increases with declining temperature, reflecting the behavior of fluids under varying thermal conditions.