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a) A slab of aluminium (2024-T6), which measures 16 cm * 16 cm x 1.5 cm, is initially at 750 K. The slab is then annealed by a water spray at 15°C, which strikes both sides of the slab. The convective heat transfer coefficient is estimated to be 1500 Wm2K-1. Neglect convection off the edges of the slab since almost all the surface area is on the two 16 cm x 16 cm sides. 1) Calculate the Biot Number and comment on its significance (5 marks] II) Determine how much time is required to cool the slab to 47°C? [7 marks] til) State and justify any assumptions made for this analysis [5 marks] Data and Formulae Properties for Aluminium (2024-T6) Density: 2770 kg m 3 Specific heat capacity: 1004 J kg-K-1 Thermal conductivity at 535K: 186 W m-K-1 Cooling time: t = pcplchar [T(t)-Too In h Ti-To b) Consider a water-water parallel flow heat exchanger. Water enters one side of a heat exchanger, with a mass flow rate of 6 kg/s at 80°Сand leaves at 50°C. Water enters the other side of the heat exchanger at 20°C and leaves at 40°C. Determine which is the hot and cold stream. Explain the answer with appropriate temperature values given in the problem. [2 marks] il) Determine the rate of heat transfer from the hot to the cold stream. [3 marks) iii) Determine the mass flow rate of the cold stream. [3 marks]