IV. Heat and Mass Transfer Properties
Conduction
in gases and liquids, _________ is due to the collisions of the molecules during their random motion
Conduction
in solids, it is due to the combination of vibrations of molecules in a lattice and the energy transport by free electrons.
Convection
mode of transfer of energy between a solid surface and the adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion
Convection
the faster the fluid motion, the greater the ________ heat transfer
Ein
total energy entering the system (eq.)
Eout
total energy leaving the system (eq.)
Conduction
transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles
adiabatic process
A process during which there is no heat transfer is called an ________, which could be either be because the system is well insulated so that only a negligible amount of heat can pass through the boundary, or both the system and the surroundings are at the same temperature and therefore there is no driving force (temperature difference) for heat transfer.
electromagnetic radiation
Above the absolute temperature of zero (K), all substances emit _________.
Basic laws of transport
All transport phenomena are the result of lack of equilibrium between parts of the system.
Molecular diffusivity
Also known as diffusion coefficient D) in Fick's law, ______ ______ depends on the diffusing molecular species, the medium in which it is diffusing and the temperature.
T
As the rate of transport can logically be expected to be proportional to the area A available to the transport, it is often customary to deal with the rate of transport per unit area (known as flux, J). (T/F)
steady
At _____ state, all the properties that define the state' of the system (temperature, pressure, chemical composition etc.) remain constant with time.
△Esystem
Change in internal, kinetic, potential, etc., energies (eq.)
kinetic and potential
Closed systems normally remain stationary during a process and thus experience no change in their ____ and _____ energies.
ΔΕ = ΔU
Common systems are stationary and do not involve any changes in their velocity or elevation during a process. Thus, the changes in kinetic and potential energies are zero and the total energy change relation is reduced to ______. (eq.)
perpendicular
Consider a composite slab formed by several layers of material, each with different k. At steady state, the heat flux through any plane ______ to the direction of heat transfer must be equal:
location (z)
Considering steady state wherein the temperature and the concentration depend only on the ______.
T
Diffusion in true solids is extremely slow. T/F
T
Emass is zero for systems that involve no mass flow across their boundaries (i.e., closed systems). (T/F)
heat, work, and mass flow.
Energy can be transferred to or from a system in three forms: (h,w,mf)
Energy transfer
Energy interactions are recognized at the system boundary as they cross it, and they represent the energy gained or lost by a system during a process.
Mechanisms of energy transfer
Energy interactions are recognized at the system boundary as they cross it, and they represent the energy gained or lost by a system during a process.
Convection
Energy is first transferred to the air layer adjacent to the surface of the block by ______.
Convection
Energy is then carried away from the surface by __________ within the air, which is due to random motion of air molecules, and the bulk or macroscopic motion of the air, which removes the heated air near the surface and replaces it by the cooler air.
(Heat Transfer, Q)
Energy transfer by heat
(Work transfer, W)
Energy transfer by work
(Mass Flow, m)
Energy transport by mass
to, increases from, decreases
Heat transfer ____ a system (heat gain) _________ the energy of the molecules and thus the internal energy of the system, and heat transfer _____ a system (heat loss) _______ it since the energy transferred out as heat comes from the energy of the molecules of the system. (in/from a system)(Increases/decreases)
J = kcΔC
If the driving force for mass transfer is expressed in terms of a difference in concentration, then the steady-state flux is written as: (eq.)
work
If the energy crossing the boundary of a closed system is not heat, it must be____.
material medium
In contrast with conduction and convection, heat transfer by radiation does not require the presence of a ___ ____.
T
In crystals and metals, molecular transport occurs mainly through defects (holes) in the crystal lattice. T/F
conduction, diffusion Fourier's, Fick's laws
In media with no considerable internal mobility (e.g. solids), heat travels by ________ and mass travels by molecular _________ and are governed by ______ and _____ laws, respectively.
Q
In other cases of transport, the quantity of whatever is being transported (mass, electric charge, momentum) would replace ______, in appropriate units (kilograms, coulombs, Newton-meters etc.).
T
In porous solids, the bulk of mass transfer occurs through the gas or liquid filling the pores and not through the solid matrix. T/F
T
In the SI system, molecular diffusivity is expressed in units of m^2/s, just as thermal diffusivity. T/F
△E = △U + △KE + △PE
In the absence of electric, magnetic, and surface tension effects i.e., for simple compressible systems), the change in the total energy of a system during a process is the sum of the changes in its internal, kinetic, and potential energies, expressed as: (eq.)
T
In the case of mass transfer in gases, the driving force is usually given in terms of a difference in partial pressures rather than concentrations. T/F
Energy balance (Ein - Eout = △Esystem)
It can be expressed as: "The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process"
Diffusivity in gases
It can be predicted quite accurately, with the help of the kinetic theory of gases. The diffusivity of gases in binary mixtures, at room temperature and atmospheric pressure is in the order of 10^-5to 10^-4 m^2/s
Energy
It can exist in numerous forms such as internal (sensible, latent, chemical, and nuclear), kinetic, potential, electric, and magnetic, and their sum constitutes the total energy E of a system.
Diffusivity in liquids
It have been predicted by models, including the Einstein-Stokes equation for the Brownian diffusion of a solute.
Thermal conductivity
It is a property of the material. It varies with temperature and is strongly pressure dependent in the case of gases.
Thermal diffusivity
It is a useful concept in heat transfer analysis. It is defined as the ratio of thermal conductivity to the 'volumetric heat capacity of the material. Volumetric heat capacity is obtained by multiplying the mass specific heat by the density
First law of thermodynamics
It is also known as the conservation of energy principle
Mass flow in and out of the system
It is an additional mechanism of energy transfer.
Heat
It is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference
Heat flux
It is just the heat rate (Q/t in Watts) divided the area (A)
notion of resistance
It is sometimes convenient to decompose the complex ______ to its physical components.
Work
It is the energy transfer associated with a force acting through a distance i.e. a rising piston, a rotating shaft, and an electric wire crossing the system boundaries
Z/K z=distance in the direction of transport , k= thermal conductivity
It is the total thermal resistance. (eq.)
Thermal radiation
It refers to electromagnetic radiation in the wavelength range of 10^-7 to 10^-4m and encompasses mainly the range of infrared radiation
Natural and Forced Convection
Kinds of convection: (2:n,f)
matter, transfer coefficient
Like Prandl number, the Schmidt number contains only properties of the _______. The Sherwood number contains the_____ _______, as does the Nusselt number in heat transfer.
Conduction
Mechanisms of heat transfer that can take place in solids, liquids, or gases
conduction, convection, and radiation
Mechanisms of heat transfer, Heat is transferred by three mechanisms: (3:c,c,r)
T
Most agricultural processes involve some sort of heat transfer. (T/F)
exchange
Most importantly, life itself depends on the _______ of material through biological membranes.
Ein - Eout
Net energy transfer by heat, work and mass (eq.)
Ein - Eout = (Qin - Qout) + (Win - Wout) + Emass,in - Emass,out) = △E system
Noting that energy can be transferred in the forms of heat, work, and mass, and that the net transfer of a quantity is equal to the difference between the amounts transferred in and out, the energy balance can be written more explicitly as ______________. (eq.)
Thermal diffusivity
Physically, _____ _____ can be interpreted as the ratio of the material's ability to transfer heat to its capacity to store heat.
steady-flow process
Processes with steady flow devices can be represented by an idealized process, called the ______ ______, which can be defined as a process during which a fluid flows through a control volume steadily. That is, the fluid properties may vary with location within the control volume, but at any fixed point they remain the same during the entire process.
T
Q is zero for adiabatic systems. (T/F)
Transmitted, reflected, absorbed
Radiation energy impinging on matter is partly_________, partly ______ and partly_______.
volumetric phenomenon
Radiation is a ______ ____ but is usually considered to be a surface phenomenon for solids that are opaque to thermal radiation such as metals, wood, and rocks since the radiation emitted by the interior regions of such material can never reach the surface, and the radiation incident on such bodies is usually absorbed within a few microns from the surface
steady flow
Several engineering devices operate for long periods of time under the same conditions, and they are classified as ______ devices.
T
The change in the total energy △E of a stationary system is identical to the change in its internal energy △U. (T/F)
10^-25m^2/s
The diffusion coefficients for small ions in solid glasses may be as low as_________. (qnty)
gradient
The driving force F is always a ______, representing the deviation from equilibrium
internal energy (U), kinetic energy (KE) and potential (PE), E = U+KE + PE
The magnetic, electric, and surface tension effects are significant in some specialized cases only and are usually ignored. In the absence of such effects, the total energy (E) of a system consists of the _______, _______, and, _______ and is expressed as _________.
heat transfer and work
The only two forms of energy interactions associated with a fixed mass or closed system are ____ and _____.
Proportional, inversely proportional
The rate of transport is _______ to the driving force and _________ to the resistance of the medium to the transport.
z/D
The same conclusion is valid for mass transfer or for any kind of transfer. In the same manner, resistance to mass transfer is defined as ____. (eq.)
Basic laws of transport
They all obey a universal law, similar to the familiar Ohm's law, which can be expressed, in general terms, as follows: (E) Voltage (I) Current (R) Resistance
T
Transport of moisture and oxygen through the package often determines the shelf life of the product within. (T/F)
Sherwood (Sh), Schmidt (Sc)
Two additional dimensionless groups are useful in mass transfer calculations:
Chemical energy
Types of internal energy associated with the atomic bonds in a molecule
Sensible energy
Types of internal energy associated with the kinetic energies of the molecules
Latent energy
Types of internal energy associated with the phase of a system
Nuclear energy
Types of internal energy tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself
intervening medium
Unlike conduction and convection, the transfer of energy by radiation does not require the presence of an _______ _____.
T
W is zero for systems that involve no work interactions. (T/F)
Increases, decreases
When mass enters a system, the energy of the system _______ because mass carries energy with it, and when some mass leaves the system, the energy contained within the system ______ because the leaving mass takes out some energy with it. (Increases/decreases)
decreases
When some hot water is removed from a water heater and replaced with same amount of cold water, the energy content of the hot water tank (the control volume) _________ as a result of mass interaction. (Increases/decreases)
Emass = mh
When the kinetic and potential energies of a fluid stream are negligible, this equation is simplified to ________.
power
Work done per unit time is called ________, denoted by W
to, increases from, decreases
Work transfer ____ a system (i.e., work done on a system) _________ the energy of the system, and work transfer ____ a system i.e., work done by the system) _________ it since the energy transferred out as work comes from the energy contained in the system. (in/from a system)(Increases/decreases)
Energy, energy E
________ can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum is the total ______ of a system.
Forced convection
a kind of convection where the fluid movement (flow) is caused by factors independent of the transfer.
Natural (or free) convection
a kind of convection where the movement is caused by heat or mass transfer itself, usually by virtue of density differences.
Radiation
all solids, liquids, and gases emit, absorb, or transmit _________ of varying degrees
First law of thermodynamics
based on experimental observations, it states that energy can be neither created nor destroyed during a process; it can only change forms.
produce work
car engines and hydraulic, steam, or gas turbines
△E system
change in total energy of the system (eq.)
consume work
compressors, pumps, and mixers
Radiation
energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.
Radiation
energy transfer by ________ is fastest (at the speed of light) and it suffers no attenuation in a vacuum.
Conduction
example: A cold canned drink placed in a warm room will eventually warm up to the room temperature as a result of heat transfer from the room to the drink through the aluminum can by conduction
Convection
example: cooling of a hot block through blowing of cool air over its top surface.
