tailieunhanh - Ebook Heat convection: Part 2

(BQ) Part 2 book "Heat convection" has contents: Heat transfer in channel flow, free convection, correlation equations - Forced and free convection, convection in microchannels. | 6 HEAT TRANSFER IN CHANNEL FLOW Introduction We consider internal flow through channels such as ducts, tubes and parallel plates. The following factors should be noted in analyzing heat transfer in internal flow. (1) Laminar vs. turbulent flow. Transition from laminar to turbulent flow takes place when the Reynolds number reaches the transition value. For flow through tubes the experimentally determined transition Reynolds number Re D t is Re Dt uD 2300 , () where D tube diameter u mean velocity kinematic viscosity (2) Entrance vs. fully developed region. Based on velocity and temperature distribution, two regions are identified: (i) Entrance region (ii) Fully developed region The length of the entrance region for velocity and temperature as well as the characteristics of these regions will be examined. 204 6 Heat Transfer in Channel Flow (3) Surface boundary conditions. Two common thermal boundary conditions will be considered: (i) Uniform surface temperature (ii) Uniform surface heat flux (4) Objective. A common problem involves a fluid entering a channel with uniform velocity and temperature. The objective in analyzing internal flow heat transfer depends on the thermal boundary condition. (i) Uniform surface temperature. In this class of problems we seek to determine axial variation of the following variables: (1) Mean fluid temperature (2) Heat transfer coefficient (3) Surface heat flux (ii) Uniform surface flux. For this class of problems the objective is to determine axial variation of the following variables: (1) Mean fluid temperature (2) Heat transfer coefficient (3) Surface temperature Hydrodynamic and Thermal Regions: General Features We consider fluid entering a channel with uniform velocity Vi and temperature Ti . Velocity and temperature boundary layers form on the inside surface of the channel. The two boundary layers grow as the distance x from the entrance is increased. Two regions are identified for each of the flow .