Heat Transfer Concepts Quiz

Fundamentals of Convection

• Convection is a heat transfer mechanism through a fluid, with bulk fluid motion.
• Classified as natural (or free) or forced convection.
• Forced convection: Fluid flow is induced by external means (e.g., pumps, fans).
• Natural convection: Fluid motion is due to buoyancy differences (warmer fluid rises, cooler fluid falls).
• Further classified as external (flow over a surface) or internal (flow in a pipe).
• Velocity and thermal boundary layers form during fluid flow.
• Dimensionless numbers (Reynolds, Prandtl, Nusselt) are crucial to analyze convection.

Physical Mechanism of Convection

• Convection involves fluid motion, enhancing heat transfer.
• Fluid motion facilitates higher rates of conduction between warmer and cooler fluid portions.
• Heat transfer via convection is typically greater than via conduction.

Classification of Fluid Flows

• Viscous Flow: Significant frictional effects.
• Internal flow: Fluid is confined by a surface (e.g. pipe).
• External flow: Fluid is not contained (e.g., fluid flow over a structure).
• Compressible flow (when density changes significantly during flow).
• Incompressible flow (when density remains fairly constant across a flow).
• Laminar flow: Smooth, orderly flow with layers.
• Turbulent flow: Disordered, chaotic flow with fluctuations.
• Steady flow: Fluid properties do not vary with time at a fixed location.
• Unsteady flow: Fluid properties vary with time at a fixed location.
• One-dimensional, two-dimensional, or three-dimensional flow: Based on the variations of velocity across the flow field.

Velocity Boundary Layer

• The no-slip condition means fluid velocity is zero at the surface of a solid boundary.
• Velocity gradient develops in the fluid layer adjacent to the wall (boundary layer).
• Viscous forces cause this velocity gradient close to the wall.

Wall Shear Stress

• Shear stress τ is the force per unit area across a surface (related to the velocity's gradient in the fluid).
• Shear stress at a surface (wall) is often expressed as a function of the dynamic viscosity of the fluid.
• The proportionality is often expressed as τw = µ(du/dy)|y=0

Thermal Boundary Layer

• The thickness of thermal boundary layer δt is similar to the velocity boundary layer's thickness.
• The temperature changes significantly within the area near the wall (thermal boundary layer).

Prandtl Number

• The Prandtl number Pr is the ratio of momentum to thermal diffusivity and is a key parameter for convective heat transfer.
• Critical Reynolds number (Recr) marks the transition from laminar to turbulent flow.

Heat and Momentum Transfer in Turbulent Flow

• Turbulent flow is characterized by fluctuating fluid motion (swirling eddies).
• In turbulent flow, heat and momentum transfer occur much more rapidly than in laminar flow.
• The higher heat transfer coefficient correlates well with turbulent flow in comparison to laminar flow.
• The turbulent nature of a flow influences many other fluid properties, including pressure drop.

Convection Equations for a Flat Plate

• The continuity, momentum, and energy equations govern convection.
• These equations are simplified for steady, incompressible laminar flow over a flat plate with constant properties to allow for solution
• The solutions usually involve view factors and symmetry rule.

Methods of Solving Convection Problems

• Direct method: The equations and boundary conditions are used directly to solve the temperature fields, or the heat transfer rate.
• Network method: The problem is viewed as an electrical circuit to solve for the unknown quantities.

Thermal Boundary Layers and Dimensionless Numbers

• Physical mechanisms for convection.
• Velocity and thermal boundary layer formation.
• Dimensionless parameters (Reynolds, Prandtl, Nusselt numbers).

Analogies Between Momentum and Heat Transfer

• Similarity between momentum and heat transfer.
• Empirical equations and correlations for convection heat transfer and friction coefficients.

Solutions of Convection Equations for a Flat Plate

• Methods to solve convection problems for a flat plate.
• The resulting relations between view factors and heat transfer.
• Application of similarity and symmetry rules.

Dimensional Analysis and Similarity

• Use of dimensionless numbers to determine the governing equations for different geometries.
• Similarity parameters, correlations, and relations in terms of these parameters.

Functional Forms of Friction and Convection Coefficients

• Deriving functional forms of friction and convection coefficients.
• Using dimensionless parameters as relevant similarity parameters
• Solution and visualization techniques for flow over flat plates and other configurations using the developed functional forms.

Natural Convection

• Natural convection: mechanism of heat transfer due to buoyancy forces.
• Grashof number, Pr, Ra, a crucial parameter to characterize natural convection.
• Examples of natural convection cooling/heating; vertical, horizontal plates, cylinders, spheres, and enclosures.

Combined Natural and Forced Convection

• Occurs simultaneous by natural and forced convection.
• The relative importance of each mode is assessed using Gr/Re².
• Assisting, opposing, and transverse flows.

Radiation Heat Transfer

• Basic nature of radiation.
• Electromagnetic spectrum and various wavelengths .
• Blackbody radiation and its properties .
• Wien's displacement law; Stefan-Boltzmann law; Planck's law .
• Spectral and total radiation intensities, irradiances, and radiosity.
• Emissivity, absorptivity, reflectivity, and transmissivity of surfaces.

Radiation Heat Transfer Between Surfaces

• View factors: quantify the fraction of radiation emitted from one surface that strikes another .
• Rules for calculating view factors (reciprocity rule, summation rule).
• Application of crossed-strings method for more complex shapes.

Radiation Heat Transfer: Black Surfaces

• Simplified analysis when surfaces are black.
• Net heat transfer rate calculations based on view factors.
• The governing equations and relations for radiation heat transfer for multiple black surfaces in an enclosure.

Radiation Heat Transfer: Gray Surfaces

• Analysis of radiation with gray surfaces.
• Role of radiosity in the analysis; the governing radiative transfer equations.

Radiation Shields

• Effect of radiation shields; properties and relations for parallel plates with radiation shields.
• Network method to solve radiation transfer in enclosures with numerous surfaces.

Radiation Effect on Temperature Measurement

• Implications of radiation on thermometer readings; relations for accurate temperature measurement in presence of thermal radiation.

Radiation Exchange with Emitting and Absorbing Gases

• Emitting and absorbing gases; characteristics, and relations.
• Gas absorptivity and emissivity considerations for various arrangements.
• Solutions of gas radiative transfer problems.

Atmospheric and Solar Radiation

• Solar energy.
• Atmospheric radiation.
• Solar spectrum.
• Factors involved in atmospheric absorption and scattering (temperature, height above sea level and humidity etc).

Test your understanding of key concepts in heat transfer, including the Nusselt number, convection, and conduction. This quiz covers essential principles that determine heat exchange efficiency and the role of temperature gradients. Challenge yourself with scenarios and definitions related to thermal dynamics.