Presentation on theme: "1 Chapter 2 Introduction to Heat Transfer 2.1 Basic Concepts 2.1.1 Conduction, convection, and radiation Heat transfer is the transfer of heat due to a."— Presentation transcript:
1 Chapter 2 Introduction to Heat Transfer 2.1 Basic Concepts 2.1.1 Conduction, convection, and radiation Heat transfer is the transfer of heat due to a temperature difference with different mechanisms: conduction, convection, and radiation. Conduction refers to heat transfer that occurs across a stationary solid or fluid in which a temperature gradient exists. Convection refers to the heat transfer that occurs across a moving fluid in which a temperature gradient exists. Radiation refers to the heat transfer between two surfaces at different temperatures separated by a medium transparent to the electromagnetic waves emitted by the surfaces.
2 2.1.2 Fourier’s law of conduction 188.8.131.52 One-dimensional Consider the conduction of heat through a slab of thickness L, as shown in Fig. 2.1-2. The lower and upper surfaces are kept at a constant temperature T 1 and T 2, respectively. A steady-state temperature profile T(y) is established in the slab. Consider two surface in slab separated with a infinitesimal distance dy, as shown in Fig. 2.1-2. due to temperature gradient generated in the slab, heat flow from the surface y to the surface y+dy. A heat flux is defined as the amount of heat transferred per unit area per unit time, and can be expressed as [2.1-1] where k is the thermal conductivity of the medium. This equation is Fourier’s law of conduction for one-dimensional heat conduction in the y-direction. The mks units of the heat flux and the thermal conductivity are W/m 2 and Wm -1 K -1, respectively.
3 The thermal conductivities of some common materials are given in Fig. 2.1-3 and Table 2.1-1. 184.108.40.206 Three-dimensional For heat transfer in a three-dimensional medium, the Fourier’s law can be expressed for each of the three coordinate directions [2.1-2][2.1-3][2.1-4] And can be expressed in a three-dimensional form of Fourier’s law of conduction. 220.127.116.11 The Thermal Diffusivity The thermal diffusivity, α, is defined as Where and C v are the density and specific heat of the material, respectively.
4 2.1.3 Thermal boundary layer Consider a fluid of uniform temperature T ∞ approaching a flat plate of constant temperature T s in the direction parallel to the plate. At the solid/liquid interface the fluid temperature is T s since the local fluid particles achieve thermal equilibrium at the interface. The fluid temperature T in the region near the plate is affected by the plate, varying from T s at the surface to T ∞ in the main stream. This region is called the thermal boundary layer.
5 Definition of thermal thickness: The thickness of thermal boundary layer δ T is taken as the distance from the plate surface at which the dimensionless temperature (T-T S )/(T ∞ -T S ) reaches 0.99. In practice it is usually specified that T=T ∞ and at y=δ T. The effect of conduction is significant only in the boundary layer. Beyond it the temperature is uniform and the effect of conduction is no longer significant. A fluid of uniform temperature T entering a circular tube of inner diameter D and uniform wall temperature T S, as illustrated in Fig. 2.1-5. A thermal boundary layer begins to develop at the entrance, gradually expanding until the layers from oppositive sides approach the centerline. This occurs at [2.1-7] [2.1-8] where (Reynolds number) (Prandtl number) Inertial force Viscous force Viscous diffusivity thermal diffusivity
6 Define average temperature [2.1-10] The thermally fully developed temperature profile in a tube is one with a dimensionless temperature (T s -T)/(T s -T av ) independent of the axial position, that is [2.1-12] 2.1.4 Heat transfer coefficient Consider the thermal boundary layer. At the solid/liquid interface heat transfer occurs only by conduction since there is no fluid motion. Therefore, the heat flux across the solid/liquid interface is [2.1-13] This equation cannot be used to calculate the heat flux when the temperature gradient is an unknown. A convenient way to avoid this program is to introduce a heat transfer coefficient, defined as follows: [2.1-14]
7 The absolute values are used to keep h always positive. From Eq. [2.1-14] The equation is called Newton’s law of cooling. For fluid flow through a tube of an inner radius R and wall temperature T S, a similar equation can be used: [2.1-16] Where T av is the average fluid temperature over the cross-sectional area R 2. Consider the thermally fully developed region shown in Fig. 2.1-5. In the case of a constant heat flux, the heat transfer coefficient h is constant in the thermally fully developed region. From Eq. [2.1-16] we see that (T S -T av ) is also constant. From this and Eq.[2.1-12], we have [2.1-17] [2.1-15] Since T S and T av are independent of r, is also independent of r, Let us consider the case of a constant wall temperature T S. Eq. [2.1-12] can be expanded and solved for to give (constant )
8 [2.1-18] Since T is dependent on r, is also dependent on r. 2.2 Overall energy-balance equation 2.2.1 Derivation Consider a control volume Ω bounded by control surface A through which a moving fluid is flowing. As defined in previous chapter, the control surface is composed by A in, A out, and A wall. Consider an infinitesimal area dA in vector form is ndA, the inward and outward heat transfer rate through area dA is -q ． ndA and q ． ndA, respectively. The energy conservation law (first law of thermodynamics) written for an open system under unsteady-state condition is [2.2-3] (constant T S )
9 Term 1: Rate of energy accumulation The thermal, kinetic, and potential energy per unit mass of the fluid are C v T, v 2 /2, and ψ, respectively, where C v, T, v are the specific heat, temperature, and velocity of the fluid. The total energy per unit mass of the fluid [2.2-4] The total energy in the differential volume element dΩis dE t =ρe t dΩ. dm =ρdΩ This can be integrated over Ω to obtain the total energy in the control volume E t E t (overall) = (integral) And the rate of energy change in Ω is Terms 2 & 3: Rate of energy in by mass inflow The inward energy flow rate is energy per unit mass, e t, times inward mass flow rate through dA and can be expressed as Since v=0 at the wall, above term can be expressed as
10 Term 4: Rate of heat transfer The heat transfer by conduction dA is Term 5: The rate of work done by the fluid in the C.V. on the surroundings, includes: (2) The rate of pressure work done To leave the C.V. through dA, the fluid has to work against the pressure of the surrounding fluid. Since the pressure force is pndA, the rate of pressure work required is dW p = pv ‧ ndA. Therefore, the rate of pressure work the fluid has to do to go through the C.V. is The rate of shaft work done by the fluid in the C.V. on the surroundings, that is, through a turbine or compressor, is W s (1) The rate of shaft work done (3) The rate of viscous work done To overcome the viscous force ‧ ndA, the rate of viscous work required is dW v = ( ‧ n) ‧ vdA. The rate of viscous work the fluid has to do is
11 Term 6: Heat generation rate The heat generation rate per unit volume, such as that due to Joule heating, phase transformation, or chemical reaction. The rate of heat generation in the differential volume element dΩ is dS=s dΩ. The heat generation in the control volume is S, Substituting the integral form of terms(1) through (6) into Eq.[2.2.3] = In most problem, including those in materials processing, the kinetic and potential energies are neglegible as compared to the thermal energy. Furthermore, the pressure, viscous and shaft work are usually negligible or even absent. As such, Eq.[2.2-5] reduces to [2.2-5] [2.2-6]
12 According to [2.2-3], Eq. [2.2-6] can be written as [2.2-7] Where E T is the thermal energy in the control volume, substituting Eq. [2.1-11] (definition of T av ) into this equation and assuming constant C v, we obtain [2.2-8] 2.2.2 Bernoulli’s Equation Consider the steady-state isothermal flow of an inviscid incompressible fluid without heat generation, heat conduction, shaft work, and viscous work. Substituting Eq. [2.2-4] into [2.2-5] and assuming uniform properties over the cross-sectional area A, we have
13 [2.2-9] Since T 1 =T 2 and ( vA) 1 =( vA) 2, Eq.[2.2-9] reduces to If the z direction is taken vertically upward, =gz, where g is the gravitational acceleration. As such, Eq/.[2.2-10], on multiplying by , becomes or simply Which is the Bernoulli equation.
14 Example 2.2.2 Conduction through cylindrical composite wall
15 Example 2.2.3 Heat transfer in fluid flow through a pipe Based on the C.V. selected in the figure Based on the definition of overall Heat transfer coefficient
16 Example 2.2.4 Counterflow heat exchanger Given: Hot stream Th 1, Th 2, m h cold stream inlet(T c2 ), outlet(T c1 ), and mass flow rate m c Overall heat transfer coefficient U, turbulent Find Q e (steady state heat exchange rate Q e in terms of U, T h and T c ). For hot stream For cold stream Therefore and [2.2-47] [2.2-48] From view of overall
17 Because we want to express Q e in terms of U, therefore, considering the C.V. in the inner pipe. Similarly, for the C.V. in the outer pipe and Subtracting Eq.[2.2-52] from Eq. [2.2-50], we have From view of C.V.
18 Integrating from z=0 to z=, and substituting [2.2-47] and [2.2-48] to [2.2-53]
19 Example 2.2.5 Heat transfer in laminar flow over a flat plate Given: Steady state, constant physical properties, no heat generation Find: T and heat transfer coefficient Approach: 1.Construct C.V. 2.Find the mass flow rate into the C.V. from surface 4 3.Consider the energy balance 4.Substituting m 4 into Eq. [2.2-57], and according to Fourier’s law of conduction
20 we have 5. Assume and 6. Assume Please see the derivation in other pages
21 7. By the definition of heat transfer coefficient From Eq.[1.4-62] From Eqs. [2.2-65], [2.2-68], and [2.2-69] We have [2.2-65][2.2-69]