thermo Flashcards
What is electromagnetic radiation, and how fast does it travel in a vacuum?
Electromagnetic radiation is emitted by matter due to changes in electron configuration. It travels at the speed of light, approximately 2.9979 × 10^8 m/s in a vacuum.
How can radiation be described in terms of quanta or photons?
Radiation can also be viewed as the propagation of discrete packets of energy called quanta or photons. Shorter wavelength radiation has larger photon energies and is more destructive.
What is thermal radiation, and how is it related to the temperature of a substance?
Thermal radiation is emitted due to the vibrational and rotational motion of electrons, atoms, and molecules of a substance. It is directly related to the temperature of the substance; all bodies above absolute zero emit thermal radiation
In which regions of the electromagnetic spectrum does thermal radiation lie?
Thermal radiation lies in the wavelength range of about 0.1 to 100 µm (10^-7 to 10^-4 m) and covers the entire infrared (IR) region, all the visible region, and part of the ultraviolet (UV) region.
Is radiation a volumetric or surface phenomenon, and how does it depend on a surface’s nature?
For semi-transparent solids, semi-transparent liquids, and gases, radiation is a volumetric phenomenon. For opaque (non-transparent) solids, radiation is a surface phenomenon. The amount of radiation emitted depends on the temperature of the body’s surface and the nature of that surface.
What is a blackbody, and what are its key properties?
A blackbody is an ideal surface that emits the maximum amount of radiation at a specified temperature and wavelength. It absorbs all radiation incident upon it and emits radiation uniformly in all directions. It is described by Stefan-Boltzmann’s law.
How does the emissivity of a surface relate to a blackbody, and what factors can affect it?
Emissivity (ε) is the ratio of the total emissive power of a surface to that of a blackbody at the same temperature. Emissivity varies between 0 and 1 and can change with temperature, wavelength, and radiation direction.
What is a grey body, and how does it differ from a blackbody?
A grey body has an emissivity (ε) less than 1 but constant for all wavelengths. Its spectral emissivity (ελ) is independent of wavelength. Grey bodies follow the Stefan-Boltzmann law for grey bodies.
Explain absorptivity, reflectivity, and transmissivity in the context of radiation.
Absorptivity (α) is the fraction of radiation absorbed by a material, reflectivity (ρ) is the fraction reflected, and transmissivity (τ) is the fraction transmitted. They satisfy the equation α + ρ + τ = 1.
What is the greenhouse effect, and how does it relate to Wien’s displacement law and radiation properties?
The greenhouse effect occurs when radiation from the sun passes through a transparent material but is trapped as long-wavelength radiation inside. It is related to Wien’s displacement law and the varying transmissivity of materials for different wavelengths. Greenhouse gases have similar transmissivity properties to glass, trapping heat on Earth.
What is Wien’s Displacement Law, and how does it relate to temperature and radiation emission?
Wien’s Displacement Law states that as the temperature of an object increases, the wavelength at which the spectral emissive power is greatest (peak of the curve) shifts to shorter wavelengths. It is directly related to the temperature of the emitting body.
How does real surface behavior differ from ideal blackbody behavior in terms of radiation properties?
Real surfaces deviate from ideal blackbody behavior. They can have varying emissivity with wavelength, direction, and temperature, which affects their radiation properties.
Explain how the greenhouse effect occurs and its relationship with the transmissivity of materials.
The greenhouse effect occurs when shorter-wavelength solar radiation passes through a material (e.g., glass or greenhouse gases) but is trapped as longer-wavelength radiation inside due to the material’s low transmissivity for those longer wavelengths.
What is irradiation, and how is it related to absorption, reflection, and transmission of radiation?
Irradiation is the radiation energy incident upon a surface per unit area and per unit time. It is related to absorption (α), reflection (ρ), and transmission (τ) of radiation, where α + ρ + τ = 1.
How does the radiation emitted by real surfaces compare to ideal blackbody behavior?
Radiation emitted by real surfaces can vary with wavelength and temperature, making them deviate from ideal blackbody behavior. Some real surfaces approach grey body behavior, while others may differ significantly.
What is the significance of the spectral blackbody emissive power, and what does it depend on?
The spectral blackbody emissive power (Ebλ) is the radiation energy emitted by a blackbody at a specific temperature per unit time, unit area, and unit wavelength. It depends on temperature and wavelength.
How does the emissivity of a surface change with temperature, wavelength, and direction?
The emissivity of a surface can vary with temperature, wavelength, and radiation direction. It is a measure of how closely a surface approximates a blackbody for specific conditions.
Describe the behavior of objects at different temperatures in terms of their radiation and color.
Objects at different temperatures emit radiation at varying wavelengths. For example, lower temperature objects emit longer-wavelength radiation and appear red, while higher temperature objects emit shorter-wavelength radiation and appear white or blue.
How do greenhouse gases contribute to the greenhouse effect on Earth?
Greenhouse gases have transmissivity properties similar to glass, allowing shorter-wavelength solar radiation to pass through but trapping longer-wavelength thermal radiation inside the Earth’s atmosphere. This contributes to the greenhouse effect.
What factors affect the emissivity of a real surface, and why is it important in radiative heat transfer?
Factors affecting emissivity include temperature, wavelength, and direction. Emissivity is crucial in radiative heat transfer because it determines how effectively a surface emits and absorbs thermal radiation.
What is fluid mechanics?
Fluid mechanics is a branch of mechanics that deals with the behavior of fluids, encompassing fluid statics (fluids at rest) and fluid dynamics (fluids in motion) and their interactions with solid boundaries.
What defines a fluid?
A fluid is a substance in either the liquid or gas phase. It’s distinguished from solids by its continuous deformation under applied shear stress.
What is the “no-slip condition” in fluid mechanics?
The no-slip condition states that fluid molecules adjacent to a solid surface adhere to the surface and have zero velocity relative to it. This condition is due to viscosity.
How can fluid flows be classified based on viscosity?
Fluid flows are classified as viscous or inviscid. Viscous flows involve significant internal frictional effects, while inviscid flows neglect viscosity’s effects, typically occurring away from solid boundaries.
What is the Reynolds number, and what does it determine?
The Reynolds number (Re) is a parameter used to determine the flow regime of a fluid, whether it’s laminar (ordered) or turbulent (chaotic). It depends on fluid density, velocity, characteristic length, and viscosity.
How can you classify fluid flows based on internal or external flow?
Fluid flows are categorized as internal or external. Internal flow occurs within confined spaces, while external flow occurs over surfaces.
What is the difference between incompressible and compressible flow?
Incompressible flow assumes that fluid density remains constant, which is accurate for most liquids. Compressible flow considers density variations, especially in high-speed gas flows.
What determines whether a fluid flow is considered steady or unsteady?
Steady flow means properties don’t change with time at a specific point, while unsteady flow indicates changes with time. Uniform flow means no changes over a specified region.
How is fluid flow classified as one-dimensional, two-dimensional, or three-dimensional?
Fluid flow can be one-, two-, or three-dimensional, depending on the velocity distribution. In practical cases, one- or two-dimensional flow models are often used for simplicity.
What is surface tension, and how does it affect liquid behavior?
Surface tension (σs) is the force per unit length acting at a liquid-gas interface due to molecular attractive forces. It causes liquids to form spherical shapes and influences phenomena like capillary rise.
What is the capillary effect in fluid mechanics?
The capillary effect is the rise (or fall) of a liquid in a small-diameter tube (capillary) inserted into a liquid. It occurs due to differences in cohesive and adhesive forces.
What is the meniscus in the context of capillary effect?
The meniscus is the curved free surface of a liquid in a capillary tube. It forms due to the balance between cohesive forces within the liquid and adhesive forces between the liquid and the solid tube.
How does the contact angle (𝜙) relate to the wetting behavior of a liquid on a solid surface?
The contact angle (𝜙) is the angle between the tangent to the liquid surface and the solid surface at the point of contact. For wetting fluids, 𝜙 is less than 90°, while for non-wetting fluids, 𝜙 is greater than 90°.
What is the formula for calculating the capillary rise (or fall) in a circular tube?
The capillary rise (or fall) in a circular tube can be calculated using the formula: ℎ = 2𝜎s𝜌𝑔𝑅 cos 𝜙, where ℎ is the rise (or fall) height, 𝜎s is surface tension, 𝜌 is density, 𝑔 is acceleration due to gravity, 𝑅 is the tube radius, and 𝜙 is the contact angle.
How does the capillary effect change with tube diameter and fluid density?
The capillary effect is inversely proportional to the tube radius, meaning thinner tubes exhibit greater capillary rise. It’s also inversely proportional to fluid density, so lighter fluids experience more significant capillary rise.
What is the significance of the Reynolds number (Re) in fluid mechanics?
The Reynolds number (Re) is a dimensionless parameter used to determine the flow regime (laminar, turbulent, or transitional) of a fluid flow. It depends on fluid density, velocity, characteristic length, and viscosity.
When is the capillary effect negligible in practice?
In practice, the capillary effect is negligible in tubes with a diameter greater than 10 mm for water and glass, making it less relevant for larger tubes.
What are surfactants, and how do they affect surface tension?
Surfactants are chemicals added to liquids to alter their properties. They can significantly reduce surface tension, making it easier for the liquid to penetrate surfaces. Soaps and detergents are examples of surfactants.
How is laminar flow characterized, and in what conditions does it typically occur?
Laminar flow is characterized by smooth, orderly fluid motion with well-defined layers. It typically occurs at low velocities (low Reynolds numbers) and is observed in highly viscous fluids like oils.
What characterizes turbulent flow, and in what conditions does it typically occur?
Turbulent flow is characterized by chaotic, disordered fluid motion. It occurs at high velocities (high Reynolds numbers) and in low-viscosity fluids like air at high speeds.
What is transitional flow, and when does it occur?
Transitional flow is a flow regime that alternates between laminar and turbulent flow. It occurs at intermediate viscosities and fluid velocities and is often seen during flow transitions.
What is uniform flow in fluid mechanics?
Uniform flow implies that fluid properties do not vary with position or location within a specified region. It is different from steady flow, which relates to the constancy of properties at a fixed point.
How can fluid flow be classified based on dimensionality?
Fluid flow can be characterized as one-dimensional, two-dimensional, or three-dimensional. One or two-dimensional flow models are often used for analysis when variations in one dimension are negligible.
What determines whether a flow is classified as viscous or inviscid?
Viscous flows are characterized by significant internal frictional effects due to viscosity, while inviscid flows have negligible viscous effects compared to pressure or inertial forces. Viscosity is responsible for viscous flows, while inviscid flows are simplified for analysis.
What is the difference between natural (unforced) and forced flow in fluid mechanics?
Forced flow is generated by external means, such as pumps or fans, while natural flow is driven by natural phenomena, like temperature-induced buoyancy effects. Forced flow is common in engineering systems, while natural flow occurs in natural processes.
Define steady flow and unsteady flow in fluid mechanics.
Steady flow means that fluid properties (velocity, pressure, temperature) remain constant at a specific location over time. Unsteady flow is the opposite, where properties change with time at a specific location.
What does fluid statics deal with?
Fluid statics deals with problems associated with fluids at rest, where there is no relative motion within the fluid.
What are the forces involved in fluid statics?
In fluid statics, the only stress to consider is normal stress due to pressure, and there are no shear stresses because the fluid is at rest.
Why is fluid statics important in engineering?
Fluid statics is essential for determining forces on submerged bodies, designing dams, hydraulic presses, and liquid storage tanks.
What is the resultant hydrostatic force on a submerged plane surface?
The resultant hydrostatic force on a submerged plane surface is equal to the average pressure over the surface multiplied by the surface area.
How is the location of the center of pressure determined?
The location of the center of pressure is determined by equating moments, taking into account the pressure distribution over the surface.
What is Archimedes’ principle?
Archimedes’ principle states that the buoyant force on an immersed body is equal to the weight of the fluid displaced by the body.
What determines the rotational stability of a submerged or floating body?
The rotational stability depends on the relative positions of the center of gravity (G) and the center of buoyancy (B), with stability occurring when G is below B.
What is the metacentric height (GM)?
The metacentric height (GM) is the vertical distance between the center of gravity (G) and the metacenter (M) of a floating vessel, which measures its stability.
How does GM affect stability in floating vessels?
A positive GM ensures stability, while a negative GM can lead to instability or capsize. Higher GM values indicate increased stability.
When does a body float on the surface of a fluid?
A body floats on the surface of a fluid when its weight is equal to the buoyant force, and its average density is less than the fluid’s density.
What is the buoyant force, and how is it related to the weight of the displaced fluid?
The buoyant force is the upward force exerted on a body immersed in a fluid, and it is equal to the weight of the fluid displaced by the body.
How does fluid density affect buoyancy?
The buoyant force is proportional to the density of the fluid. In denser fluids, the buoyant force is greater.
What is stability in the context of floating bodies?
Stability of a floating body depends on the relative positions of the center of gravity (G) and the center of buoyancy (B). It’s stable if G is below B.
Explain the metacentric height (GM) and its significance.
The metacentric height (GM) is the vertical distance between the center of gravity (G) and the metacenter (M) in a floating vessel. A positive GM indicates stability.
What happens if the metacentric height (GM) is negative?
A negative GM can lead to instability and capsize of a floating vessel, especially in rough seas or unbalanced loading.
How is stability related to the metacentric height (GM)?
Higher values of GM indicate increased stability, while lower or negative GM values indicate decreased stability or instability.
What is the significance of Archimedes’ principle in buoyancy?
Archimedes’ principle states that the buoyant force on an immersed body is equal to the weight of the fluid displaced, providing a fundamental understanding of buoyancy.
Why is fluid statics important in engineering applications?
Fluid statics is crucial for designing structures like dams, hydraulic presses, and ensuring the stability of floating vessels, including ships and submarines.
How does fluid statics apply to submerged bodies in a liquid?
Fluid statics helps determine the forces acting on submerged bodies due to hydrostatic pressure and provides insights into their stability.
What is the role of fluid density in buoyancy?
The buoyant force is directly proportional to the density of the fluid. In denser fluids, the buoyant force is greater, affecting the stability of floating bodies.
What is the Bernoulli Equation?
The Bernoulli Equation is a relationship between pressure, velocity, and elevation, applicable in regions of steady, incompressible flow with negligible frictional forces.
What does the Bernoulli Equation represent?
It represents the conservation of kinetic, potential, and flow energies per unit mass of fluid along a streamline.
What are the requirements for using the Bernoulli Equation?
The flow must be steady, incompressible, have negligible viscous effects, no shaft work, and negligible heat transfer.
What is stagnation pressure?
Stagnation pressure is the sum of static and dynamic pressures and represents the pressure at a point where the fluid comes to a complete stop isentropically.
What are the limitations of the Bernoulli Equation?
Limitations include the need for steady flow, negligible viscous effects, no shaft work, incompressible flow, negligible heat transfer, flow along a streamline, and the absence of vorticity.
What is steady flow in the context of the Bernoulli Equation?
Steady flow implies that there are no changes in flow properties with respect to time at a specific location. It is a requirement for applying the Bernoulli Equation.
What does it mean for flow to be incompressible according to the Bernoulli Equation?
Incompressible flow means that the density ((\rho)) of the fluid remains approximately constant throughout the flow. It is a condition for applying the Bernoulli Equation.
Why should there be no shaft work involved in the flow for the Bernoulli Equation to be valid?
The Bernoulli Equation does not account for work interactions with devices like fans, turbines, or pumps. It is applicable when there is no exchange of energy with such machines in the flow section.
What is the significance of the constant (C) in the Bernoulli Equation?
While the Bernoulli Equation strictly applies along a single streamline, for irrotational flow with no vorticity, the constant (C) is the same for all streamlines, allowing the equation to be applied across streamlines.
What is meant by neglecting heat transfer in the context of the Bernoulli Equation?
Neglecting heat transfer means that there are no significant temperature changes in the flow section. The Bernoulli Equation assumes no heat interactions with the flow field.
What is stagnation pressure, and how does it relate to the Bernoulli Equation?
Stagnation pressure ((P_{\text{stag}})) is the sum of static and dynamic pressures. It represents the pressure at a point where the fluid is brought to a complete stop isentropically. The Bernoulli Equation uses this concept to describe the energy state of the fluid.
What are static, dynamic, and stagnation pressures in the Bernoulli Equation?
- Static pressure ((P)) represents the actual thermodynamic pressure of the fluid and does not incorporate dynamic effects.
- Dynamic pressure accounts for the pressure rise when a moving fluid is brought to a sudden stop.
- Stagnation pressure is the sum of static and dynamic pressures, representing the pressure at a point where the fluid comes to a complete stop.
How does the Bernoulli Equation relate to the conservation of mechanical energy?
The Bernoulli Equation can be seen as the conservation of mechanical energy principle for fluid flow, assuming no changes to the thermal (internal) energy of the flow stream. It represents the balance between kinetic, potential, and flow energies.
Why is the Bernoulli Equation commonly used in practice despite its limitations?
The Bernoulli Equation is a simple but powerful tool in fluid mechanics. It is used because it provides reasonable accuracy for a variety of practical fluid flow problems, even though it has some limitations. A more general energy equation accounts for additional effects and is covered in advanced courses.
