An air duct heater consists of an aligned array of electrical heating elements in which the longitudinal and transverse pitches are SL = ST = 24 mm. There are 3 rows of elements in the flow direction (NL = 3) and 4 elements per row (NT = 4). Atmospheric air with an upstream velocity of 12 m/s and a temperature of 25°C moves in cross flow over the elements, which have a diameter of 12 mm, a length of 250 mm, and are maintained at a surface temperature of 350°C. (a) Determine the total rate of heat transfer to the air and the temperature of the air leaving the duct heater. (b) Determine the pressure drop across the element bank and the fan power requirement. (c) Compare the average convection coefficient obtained in your analysis with the value for an isolated (single) element. Explain the difference between the results. (d) What effect would increasing the longitudinal and transverse pitches to 30 mm have on the exit temperature of the air, the total heat rate, and the pressure drop?

An air duct heater consists of an aligned array of electrical heating elements in which the longitudinal and transverse pitches are SL = ST = 24 mm. There are 3 rows of elements in the flow direction (NL = 3) and 4 elements per row (NT = 4). Atmospheric air with an upstream velocity of 12 m/s and a temperature of 25°C moves in cross flow over the elements, which have a diameter of 12 mm, a length of 250 mm, and are maintained at a surface temperature of 350°C. (a) Determine the total rate of heat transfer to the air and the temperature of the air leaving the duct heater. (b) Determine the pressure drop across the element bank and the fan power requirement. (c) Compare the average convection coefficient obtained in your analysis with the value for an isolated (single) element. Explain the difference between the results. (d) What effect would increasing the longitudinal and transverse pitches to 30 mm have on the exit temperature of the air, the total heat rate, and the pressure drop?

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter5: Analysis Of Convection Heat Transfer

Section: Chapter Questions

Problem 5.66P

See similar textbooks

Similar questions

5.6 A fluid flows at 5 over a wide, flat plate 15 cm long. For each from the following list, calculate the Reynolds number at the downstream end of the plate. Indicate whether the flow at that point is laminar, transition, or turbulent. Assume all fluids are at 40°C. (a) air, (b) , (c) water, (d) engine oil.
Air at 22˚C and at atmospheric pressure flows over a flat plate at a velocity of 1.65 m/s. If the length of the plate is 2.179 m and its temperature is 98 ˚C, Calculate Heat rate by using exact and approximate methods both. What is the %age difference of the heat transfer rate values by these methods? Take width of the plate as unity. Properties given at 60˚C are as follows: Density: 1.058 kg/m3 , cp = 1.005 kJ/kg˚C, k= 0.02897 w/m˚C, Kinematic viscosity is 18.97 × 10-6 m2 /s
A thin, flat plate of length L = 1 m seperates two airstreams that are in parallel flow over opposite surfaces of the plate. One airstream has a temperature of T,1=200°C and a velocity of uo,1= 60 m/s, while the other airstream has a temperature of T,2= = 25°C and a velocity 10 m/s. What is the heat flux between the two streams at the mid point of the of u0,2 plate?
Heat Transfer Convection heat transfer 7- Air at 1 atm and 27 C blows across a large concrete surface 15 m wide and unit length maintained at 55•C. The flow velocity is 4.5 m/s. Calculate Reynold number and the convection heat loss from the surface. V = 17.94 x 106 k = 0.0273 Pr = 0.7 Nu, = 2604.39 Answers: Re = 3.76 x 106 and q = 3980W %3D
Metallic cylinder 10 mm in diameter maintained at 50 °C with the engine oil fluid in a cross flow over the cylinder for which the air temperature To is 20°C and the velocity of 5 m/s (see Fig.) 1. What are the main assumptions? 2. Calculate the rate of heat transfer per unit length? 3. Discuss the above results? Engine Oil Ts = 50 °C Too = 20 °C D = 0.01 m U∞ = 5 m/s Thermophysical Properties of Oil Engine 2 T P Cp M-10² v.106 k-10³ (K) (kg/m³) (kJ/kg-K) a 10 (m²/s) B-10³ (K-¹) (N-s/m²) (m²/s) (W/m - K) Pr Engine Oil (Unused) 273 899.1 1.796 385 147 0.910 47,000 0.70 280 895.3 1.827 217 144 0.880 27,500 0.70 290 890.0 1.868 145 0.872 12,900 0.70 300 884.1 1.909 145 0.859 6400 0.70 310 877.9 1.951 145 0.847 3400 0.70 320 871.8 1.993 0.823 1965 0,70 330 865.8 2.035 8.36 96.6 0.800 1205 0.70 340 859.9 2.076 5.31 61.7 139 0.779 793 0.70 Convection Heat Transfer Correlations for External Flow Correlation Range of Applicability 5/8 4/5 0.62 Re2Pr1/3 ᏂᎠ k =0.3+ Rep 282,000 ReD Pr > 0.2 1/4…
Q1/ Consider the following fluids, each with a velocity of u = 9 m/s and a temperature of 10°C, in cross flow over a 10-mm diameter cylinder maintained at 50°C. Calculate the rate of heat transfer per unit length for saturated water? ***** 2 Satu Dety PP Liquid 3149 9920 4.246 996.0 528 9040 7.354 902.1 393 9901 Nu 0.3+1 Endalay Specific Heal Veputation 0.62 Re2 Pro [1 + (0.4/Pypa[1+ (282,000) aper 0.0231 2402 4180 00304 4378 0.0397 2419 4178 0.0612 2407 4179 00655 2335 4180 tree Thermal Coductivity THAY Liquid Vapor Le Dynamic Vacosity 1870 Vapor Liquid 0607 0.0186 0110 0.90710 1875 0615 0.0169 0.796 101 100110 1880 BAZ3 0.0192 0720x10 1016 10 1825 0.631 0.5196 0653x10 103150 1832 0437 0.0000 0596x10104610 Fran www. Lignon A'IN Ligad Vaper 614 1.00 0.24710 542 100 0234-10 4.83 1.00 0.337x10 4.32 100 0377x10 191 100 0415-10
A vehicle carrying refrigeration box is moving with a speed of 24 m/s on a road withambient temperature of 323 K. The dimensions of box are 3 m(H) × 4 m (W) × 10 m (L).Assume a wall temperature of 283 K and air flow is parallel to the longest side of the box.Neglecting the heat loss from the front side and backside of the box, calculate heat lossfrom four surfaces. For flow over flat surfaces useNu = 0.036(Re)0.8(Pr)1/3The properties of air at bulk mean temperature are : = 1.165 kg/m3, Pr = 0.701, cp = 1005 J/kg – K,  = 16 × 10–6m2/s
An air heater may be fabricated by coiling Nichrome wire and passing air in cross flow over the wire. Consider a heater fabricated from wire of diameter D = 1 mm, electrical resistivity pe = 106 Q-m, thermal conductivity k = 25 W/m-K, and emissivity & = 0.2. The heater is designed to deliver air at a temperature of T = 350°C under flow conditions that provide a convection coefficient of h = 350 W/m²-K for the wire. The temperature of the housing that encloses the wire and through which the air flows is Tsur = 50°C. (a) If the maximum allowable temperature of the wire is Tmax = 1200°C, what is the maximum allowable electric current I? If the maximum available voltage is AE = 110 V, what is (b) the corresponding length L of wire that may be used in the heater and (c) the power rating of the heater? Hint: In your solution, assume negligible temperature variations within the wire, but after obtaining the desired results, assess the validity of this assumption. Wire (D, L,Pe, k,&, Tmax) ↑1…
Q2/ cylindrical electrical heating element (dissipated 1000 W/m) of diameter D=10 mm, thermal conductivity k=240 W/m K, density 2700 kg/m3, and specific heat cp = 900 J/kg K is installed in a duct for which air moves in cross flow over the heater at a temperature and velocity of 27°C and 10 m/s, respectively. calculate the surface temperature per unit length of the heater.
An insulated steam pipe is used to transport high-temperature steam from one building to another. The pipe has a diameter of 500 mm, a surface temperature of 164 °C and is exposed to ambient air at -10 °C. The pipe is subjected to an extermal forced convection as the air moves in a cross-flow over the pipe at a speed of 5 m/s (see figure below). a. What are the main assumptions? b. What is the heat loss per unit length of pipe without insulation? c. What is the heat loss per unit length of pipe if the insulation thickness is 10 mm? V = 5 m/s Too= -10 °C Air Tsi = 150 °C Insulation k, = 0.026 W/m-K D = 500 mm Ts.o Os8s 50 mm Steam Thermophysical Properties of Air at Atmospheric Pressure' k 10 4.107 (N-s/m) v• 10 (m/s) a 10 (m/s) T (K) (kg/m') (kJ/kg - K) (W/m K) Pr 100 3.5562 1.032 71.1 2.00 9.34 2.54 0.786 150 2.3364 1.012 103.4 4.426 13.8 5.84 0.758 200 1.7458 1.007 132.5 7.590 18.1 10.3 0.737 250 1.3947 1.006 159.6 11.44 22.3 15.9 0.720 300 1.1614 1.007 184.6 15.89 26.3 22.5 0.707…
no previous attempt please
Consider two cases involving parallel flow of dry air at V = 5 m/s, T = 45°C, and atmospheric pressure over an isothermal plate at T = 20°C. In the first case, Rex, = 5 x 105, while in the second case the flow is tripped to a turbulent state at x = 0 m. At what x-location, in m, are the thermal boundary layer thicknesses of the two cases equal? What are the local heat fluxes, in W/m², at this location for the two cases? x = d'am = qturb = Mc i m W/m² W/m²