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Lectures by Ahmed Alhusseny

Chapter Four Problems Solutions

Heat exchangers are devices that assist the exchange of heat between two fluids at different temp... more Heat exchangers are devices that assist the exchange of heat between two fluids at different temperatures while keeping them from mixing with each other. Heat exchangers are commonly used in practice in a wide range of applications, from heating and airconditioning systems in a household, to chemical processing and power production in large plants. Note1-Heat exchangers differ from mixing chambers in that they do not allow the two fluids involved to mix. Note2-Heat transfer in a heat exchanger usually involves convection in each fluid and conduction through the wall separating the two fluids. 7.2-Types of Heat Exchangers Different heat transfer applications require different types of hardware and different configurations of heat transfer equipment. The attempt to match the heat transfer hardware to the heat transfer requirements has resulted in various types of heat exchanger designs. a-Double-Pipe Heat Exchanger This type can be considered the simplest type of heat exchanger and consists of two concentric pipes of different diameters, as shown in Fig.(7.1), where the first fluid will flow through the smaller pipe while the other fluid will flow through the annular space between the two pipes. Note-Two types of flow arrangement are possible in a double-pipe heat exchanger: in parallel flow, both the hot and cold fluids enter the heat exchanger at the same end and move in the same direction. In counter flow, on the other hand, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions. Fig.(7.1) Different flow regimes and associated temperature profiles in a double-pipe heat exchanger b-Compact Heat Exchanger It is another type of heat exchanger, which is specifically designed to realize a large heat transfer surface area per unit volume, where the ratio of the heat transfer surface area of a heat exchanger to its volume is called the area density β. A heat exchanger with β > 700 m 2 /m 3 (or 200 ft 2 /ft 3) is classified as being compact. As example of compact type is the car radiator (β ≈ 1000 m 2 /m 3). Note1-The large surface area in compact heat exchangers is obtained by attaching closely spaced thin plate or corrugated fins to the walls separating the two fluids. Note2-Compact heat exchangers are commonly used in gas-togas and gas-to-liquid (or liquid-togas) heat exchangers to counteract the low heat transfer coefficient associated with gas flow with increased surface area.

Chapter One Basics of Heat Transfer 1.1.1-Heat and Other Forms of Energy

Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, m... more Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, magnetic, chemical, and nuclear, and their sum constitutes the total energy E or (e on a unit mass basis). The forms of energy related to the molecular structure of a system and the degree of the molecular activity are referred to as the microscopic energy. The sum of all microscopic forms of energy is called the internal energy of system, and is denoted U (or u on a unit mass basis). Internal energy may be viewed as the sum of kinetic and potential energies of the molecules. The portion of the internal energy of a system associated with the kinetic energy of the molecules is called sensible energy or sensible heat, where the internal energy associated with the phase change of a system is called latent energy or latent heat. In the analysis of a system that involve fluid flow as shown in Fig.(1.1), a combination of properties u and Pv is obtained to defined by a property called enthalpy h, where; h=u+Pv and the term Pv represents the flow energy of the fluid (also called the flow work). Fig.(1.1) The internal energy (u) represents the microscopic energy of a non-flowing fluid, whereas enthalpy (h) represents the microscopic energy of a flowing-fluid

Fig.(8.1) A hot object in a vacuum chamber loses heat by radiation only 1-It does not require the... more Fig.(8.1) A hot object in a vacuum chamber loses heat by radiation only 1-It does not require the presence of a material medium to take place. 2-In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. 3-Also, radiation transfer occurs in solids as well as liquids and gases. In most practical applications, all three modes of heat transfer occur concurrently at varying degrees. But heat transfer through an evacuated space can occur only by radiation. For example, the energy of the sun reaches the earth by radiation. 4-Heat transfer by conduction or convection takes place in the direction of decreasing temperature; that is, from a high-temperature medium to a lower-temperature one. It is interesting that radiation heat transfer can occur between two bodies separated by a medium colder than both bodies. For example, solar radiation reaches the surface of the earth after passing through cold air layers at high altitudes. 8.2-Blackbody Radiation The A blackbody is defined as a perfect emitter and absorber of radiation. A blackbody absorbs all incident radiation, regardless of wavelength and direction. Also, a blackbody emits radiation energy uniformly in all directions per unit area normal to direction of emission. Fig.(8.2). That is, a blackbody is a diffuse emitter. The term diffuse means "independent of direction". The radiation energy emitted by a blackbody per unit time and per unit surface area can be determined by Stefan-Boltzmann law as follow; Eb = σ T 4 (W / m 2) …(8.1) where σ = 5.67 ×10-8 (W/m 2 · K 4) is the Stefan-Boltzmann constant and T is the absolute temperature of the surface in (K°) and Eb is called the blackbody emissive power. 8.3-Radiative Properties 8.3.1-Emissivity The emissivity of a surface represents the ratio of the radiation emitted by the surface at a given temperature to the radiation emitted by a blackbody at the same temperature. The emissivity of a surface is denoted by ε, and it takes the range of; 0 ≤ ε ≤ 1, as shown in Fig.(8.3) So, the emissivity is a measure of how closely a surface approximates a blackbody, for which ε = 1, for a whit body surface ε = 0 and for gray bodies it lies between 0 to 1. Fig.(8.3) Typical ranges of emissivity for various materials Fig.(8.2) A blackbody is said to be a diffuse emitter since it emits radiation energy uniformly in all directions 8.

1-It does not require the presence of a material medium to take place. 2-In fact, energy transfer... more 1-It does not require the presence of a material medium to take place. 2-In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. 3-Also, radiation transfer occurs in solids as well as liquids and gases.

Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, m... more Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, magnetic, chemical, and nuclear, and their sum constitutes the total energy E or (e on a unit mass basis). The forms of energy related to the molecular structure of a system and the degree of the molecular activity are referred to as the microscopic energy. The sum of all microscopic forms of energy is called the internal energy of system, and is denoted U (or u on a unit mass basis).

Papers by Ahmed Alhusseny

kufa Journal of Engineering

The experimental investigations of rotating heat exchangers are usually too costly and provide li... more The experimental investigations of rotating heat exchangers are usually too costly and provide limited understanding for the phenomena of heat and fluid flow within them; hence, a less expensive and more comprehensive method is required to investigate what can affect their overall performance. In the current study, a porous media concept is presented as an alternative way to numerically analyse the fluid flow and heat transport through a rotary thermal regenerator. An aluminum core formed of multi-packed square passages is simulated as a porous medium of an orthotropic porosity in order to allow the counter-flowing streams to flow in a way similar to that inside the regenerator core. The geometric properties of the core were transformed into the conventional porous media parameters such as the permeability and inertial coefficient based on empirical equations; so, the core has been dealt with as a porous medium of known features. Fluid and solid phases are assumed to be in a local t...

Cooling of high-performance electronic equipment using graphite foam heat sinks

Applied Thermal Engineering

Abstract In the present research, highly-conductive graphite foams have been employed to effectiv... more Abstract In the present research, highly-conductive graphite foams have been employed to effectively dissipate the heat generated in electronic components. The heat sinks suggested have been configured from staggered foamed-baffles arranged either in parallel or perpendicular to the air paths through the slots in between to reduce the pressure drop resulted while improving the heat dissipation. The performance of the currently proposed heat sinks has been examined numerically based on the volume averaging concept of porous media, with employing the local thermal non-equilibrium model to account for the interstitial heat exchange between the foam solid matrix and the fluid particles flowing across. The Simcenter STAR-CCM+ CFD commercial code has been utilised to implement the iterative solution based on the SIMPLE algorithm. A wide range of design parameters have been tested including the heat sink configuration along with geometrical characteristics of the graphite foam used. The impact of operating conditions, including the inlet airflow strength and the heat flux applied, has been inspected as well. The currently proposed heat sinks have been found efficient to meet the extremely thermal demands of high-performance electronic equipment and sweep away the heat generated there with a reasonable cost of pressure drop, where hot spots can be eliminated entirely with proper manipulation of design conditions.

Rotating metal foam structures for performance enhancement of double-pipe heat exchangers

International Journal of Heat and Mass Transfer, 2017

Chapter Four Problems Solutions

Heat exchangers are devices that assist the exchange of heat between two fluids at different temp... more Heat exchangers are devices that assist the exchange of heat between two fluids at different temperatures while keeping them from mixing with each other. Heat exchangers are commonly used in practice in a wide range of applications, from heating and airconditioning systems in a household, to chemical processing and power production in large plants. Note1-Heat exchangers differ from mixing chambers in that they do not allow the two fluids involved to mix. Note2-Heat transfer in a heat exchanger usually involves convection in each fluid and conduction through the wall separating the two fluids. 7.2-Types of Heat Exchangers Different heat transfer applications require different types of hardware and different configurations of heat transfer equipment. The attempt to match the heat transfer hardware to the heat transfer requirements has resulted in various types of heat exchanger designs. a-Double-Pipe Heat Exchanger This type can be considered the simplest type of heat exchanger and consists of two concentric pipes of different diameters, as shown in Fig.(7.1), where the first fluid will flow through the smaller pipe while the other fluid will flow through the annular space between the two pipes. Note-Two types of flow arrangement are possible in a double-pipe heat exchanger: in parallel flow, both the hot and cold fluids enter the heat exchanger at the same end and move in the same direction. In counter flow, on the other hand, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions. Fig.(7.1) Different flow regimes and associated temperature profiles in a double-pipe heat exchanger b-Compact Heat Exchanger It is another type of heat exchanger, which is specifically designed to realize a large heat transfer surface area per unit volume, where the ratio of the heat transfer surface area of a heat exchanger to its volume is called the area density β. A heat exchanger with β > 700 m 2 /m 3 (or 200 ft 2 /ft 3) is classified as being compact. As example of compact type is the car radiator (β ≈ 1000 m 2 /m 3). Note1-The large surface area in compact heat exchangers is obtained by attaching closely spaced thin plate or corrugated fins to the walls separating the two fluids. Note2-Compact heat exchangers are commonly used in gas-togas and gas-to-liquid (or liquid-togas) heat exchangers to counteract the low heat transfer coefficient associated with gas flow with increased surface area.

Chapter One Basics of Heat Transfer 1.1.1-Heat and Other Forms of Energy

Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, m... more Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, magnetic, chemical, and nuclear, and their sum constitutes the total energy E or (e on a unit mass basis). The forms of energy related to the molecular structure of a system and the degree of the molecular activity are referred to as the microscopic energy. The sum of all microscopic forms of energy is called the internal energy of system, and is denoted U (or u on a unit mass basis). Internal energy may be viewed as the sum of kinetic and potential energies of the molecules. The portion of the internal energy of a system associated with the kinetic energy of the molecules is called sensible energy or sensible heat, where the internal energy associated with the phase change of a system is called latent energy or latent heat. In the analysis of a system that involve fluid flow as shown in Fig.(1.1), a combination of properties u and Pv is obtained to defined by a property called enthalpy h, where; h=u+Pv and the term Pv represents the flow energy of the fluid (also called the flow work). Fig.(1.1) The internal energy (u) represents the microscopic energy of a non-flowing fluid, whereas enthalpy (h) represents the microscopic energy of a flowing-fluid

Fig.(8.1) A hot object in a vacuum chamber loses heat by radiation only 1-It does not require the... more Fig.(8.1) A hot object in a vacuum chamber loses heat by radiation only 1-It does not require the presence of a material medium to take place. 2-In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. 3-Also, radiation transfer occurs in solids as well as liquids and gases. In most practical applications, all three modes of heat transfer occur concurrently at varying degrees. But heat transfer through an evacuated space can occur only by radiation. For example, the energy of the sun reaches the earth by radiation. 4-Heat transfer by conduction or convection takes place in the direction of decreasing temperature; that is, from a high-temperature medium to a lower-temperature one. It is interesting that radiation heat transfer can occur between two bodies separated by a medium colder than both bodies. For example, solar radiation reaches the surface of the earth after passing through cold air layers at high altitudes. 8.2-Blackbody Radiation The A blackbody is defined as a perfect emitter and absorber of radiation. A blackbody absorbs all incident radiation, regardless of wavelength and direction. Also, a blackbody emits radiation energy uniformly in all directions per unit area normal to direction of emission. Fig.(8.2). That is, a blackbody is a diffuse emitter. The term diffuse means "independent of direction". The radiation energy emitted by a blackbody per unit time and per unit surface area can be determined by Stefan-Boltzmann law as follow; Eb = σ T 4 (W / m 2) …(8.1) where σ = 5.67 ×10-8 (W/m 2 · K 4) is the Stefan-Boltzmann constant and T is the absolute temperature of the surface in (K°) and Eb is called the blackbody emissive power. 8.3-Radiative Properties 8.3.1-Emissivity The emissivity of a surface represents the ratio of the radiation emitted by the surface at a given temperature to the radiation emitted by a blackbody at the same temperature. The emissivity of a surface is denoted by ε, and it takes the range of; 0 ≤ ε ≤ 1, as shown in Fig.(8.3) So, the emissivity is a measure of how closely a surface approximates a blackbody, for which ε = 1, for a whit body surface ε = 0 and for gray bodies it lies between 0 to 1. Fig.(8.3) Typical ranges of emissivity for various materials Fig.(8.2) A blackbody is said to be a diffuse emitter since it emits radiation energy uniformly in all directions 8.

1-It does not require the presence of a material medium to take place. 2-In fact, energy transfer... more 1-It does not require the presence of a material medium to take place. 2-In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. 3-Also, radiation transfer occurs in solids as well as liquids and gases.

Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, m... more Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, magnetic, chemical, and nuclear, and their sum constitutes the total energy E or (e on a unit mass basis). The forms of energy related to the molecular structure of a system and the degree of the molecular activity are referred to as the microscopic energy. The sum of all microscopic forms of energy is called the internal energy of system, and is denoted U (or u on a unit mass basis).

kufa Journal of Engineering

The experimental investigations of rotating heat exchangers are usually too costly and provide li... more The experimental investigations of rotating heat exchangers are usually too costly and provide limited understanding for the phenomena of heat and fluid flow within them; hence, a less expensive and more comprehensive method is required to investigate what can affect their overall performance. In the current study, a porous media concept is presented as an alternative way to numerically analyse the fluid flow and heat transport through a rotary thermal regenerator. An aluminum core formed of multi-packed square passages is simulated as a porous medium of an orthotropic porosity in order to allow the counter-flowing streams to flow in a way similar to that inside the regenerator core. The geometric properties of the core were transformed into the conventional porous media parameters such as the permeability and inertial coefficient based on empirical equations; so, the core has been dealt with as a porous medium of known features. Fluid and solid phases are assumed to be in a local t...

Cooling of high-performance electronic equipment using graphite foam heat sinks

Applied Thermal Engineering

Abstract In the present research, highly-conductive graphite foams have been employed to effectiv... more Abstract In the present research, highly-conductive graphite foams have been employed to effectively dissipate the heat generated in electronic components. The heat sinks suggested have been configured from staggered foamed-baffles arranged either in parallel or perpendicular to the air paths through the slots in between to reduce the pressure drop resulted while improving the heat dissipation. The performance of the currently proposed heat sinks has been examined numerically based on the volume averaging concept of porous media, with employing the local thermal non-equilibrium model to account for the interstitial heat exchange between the foam solid matrix and the fluid particles flowing across. The Simcenter STAR-CCM+ CFD commercial code has been utilised to implement the iterative solution based on the SIMPLE algorithm. A wide range of design parameters have been tested including the heat sink configuration along with geometrical characteristics of the graphite foam used. The impact of operating conditions, including the inlet airflow strength and the heat flux applied, has been inspected as well. The currently proposed heat sinks have been found efficient to meet the extremely thermal demands of high-performance electronic equipment and sweep away the heat generated there with a reasonable cost of pressure drop, where hot spots can be eliminated entirely with proper manipulation of design conditions.

Rotating metal foam structures for performance enhancement of double-pipe heat exchangers

International Journal of Heat and Mass Transfer, 2017

Effects of centrifugal buoyancy on developing convective laminar flow in a square channel occupied with a high porosity fibrous medium

International Journal of Heat and Mass Transfer, 2015

An effective engineering computational procedure to analyse and design rotary regenerators using a porous media approach

International Journal of Heat and Mass Transfer, 2016

A numerical study of double-diffusive flow in a long rotating porous channel

Heat and Mass Transfer, 2014

Developing convective flow in a square channel partially filled with a high porosity metal foam and rotating in a parallel-mode

International Journal of Heat and Mass Transfer, 2015

Hydrodynamically and thermally developing flow in a rectangular channel filled with a high porosity fiber and rotating about a parallel axis

International Communications in Heat and Mass Transfer, 2015

Computational Simulation of the Heat and Fluid Flow through a Rotary Thermal Regenerator Based on a Porous Media Approach,". 8th ICCHMT, Istanbul, Turkey