Layers of 23-cm-thick meat slabs (k = 0.47 w/m·°c and α = 0.13*10-6 m2 /s) initially at a uniform temperature of 7°c are to be frozen by refrigerated air at -30°c flowing at a velocity of 1.4 m/s. the average heat transfer coefficient between the meat and the air is 20 w/m2 ·°c. assuming the size of the meat slabs to be large relative to their thickness, determine how long it will take for the center temperature of the slabs to drop to -18°c. also, determine the surface temperature of the meat slab at that time. (25 points)