The following work deals with numerical investigation of a storage tank equipped with a mantle annular cavity surrounding it. Heat is transferred via the working fluid flowing in the mantle side to the inner storage tank. Water at constant temperature of 60 oC enters the mantle to heat the tank and exits at a lower temperature. The study is carried out using the commercial software Ansys−Fluent to track the change of water temperature in the tank and mantle side during the simulated period of 60 minutes. Temperature contours of tank water and mantle water are drawn at several time intervals during the simulated period. The effects of changing working fluid mass flow rate were taken into account through three values, namely; 0.0077, 0.015 and 0.02 kg/s. Three values of mantle gap thickness; 10, 15 and 20 mm, and three values of mantle length; 250, 300 and 350 mm were applied in the simulation program and their effects were monitored and displayed. Results show that increasing the value of mass flow rate causes more heat transfer rate to the storage tank. The gap thickness of 10 mm which is the smallest value among the three applied values gave the best heat transfer rate and the mantle length of 300 mm was the optimum among the applied values. The heat transfer rate to the tank was studied in terms of tank mean temperature. The maximum mean tank temperature achieved at the end of the simulated period was 42 oC.