1. Consider a co-current (parallel) flow heat exchanger which heats cold air using hot air as shown in Fig. 1. The heat exchanger is composed of 101 plates, whose top and

bottom surfaces are thermally insulated. Inlet temperatures of the hot air and cold air are 320 and 20 °C, respectively. The heat exchanger can be regarded as a steady flow system under constant pressure without heat loss. All the design parameters are described in Fig. 1. For simplicity, physical properties shown in Table 1 can be used for both hot and cold air flows. Answer the following questions. Pitch 0.003 m (including wall thickness) Cold air inlet temp. = 20 °C Hot air inlet temp. = 320 °C Mass flow 0.1 rate temperature temperature [kg/s] [°C] [°C] 20 Air (Same properties can be used for both hot and cold sides) Cold air Hot air Air inlet inlet density 320 Fig. 1. Co-current (parallel) flow air-air heat exchanger. 0.3 m Air viscosity 0.3 m Table 1. Flow conditions and physical properties. Pa H₂ [kg/m³] [Pa's] 0.7960 2.465x10-5 Wall thickness 1=0.0005 m 0.3 m 0.03583 Air thermal Air conductivity specific A₂ heat [W/(m*K)] Cpa [J/kgK] 1020.0 Wall (SUS304) Wall thickness t [m] 0.0005 Thermal conductivity Aw [W/(m*K)] 18.7/nFill in the blanks in Table 2. Nusselt number of fully developed channel flow can be assumed: (1) Nu = hd = 8.23, 1₂ where di is the hydraulic diameter. The heat exchanger effectiveness of co-current (parallel) flow is given as: Heat transfer area of each air flow side A [m²] E= Hydraulic diameter du [m] 1-exp[-(1+R) NTU]. 1+ R Table 2. Heat exchanger parameters. Heat transfer Overall heat transfer coefficient K [W/(m²*K)] coefficient h [W/(m²*K)] Heat capacity ratio R Number of transfer units NTU (2) Heat exchanger effectiveness E Exchanged heat Q[W]