Introduction

The generalization of the experimental and calculated data of the authors with the data of other studies on the efficiency of heat exchangers of thermal power plants showed that the process of heat transfer in condensers, network water heaters and apparatuses of the regeneration system of steam turbine plants is in most cases limited by heat transfer from the steam side. The difference in the levels of the heat transfer coefficient from the steam and water sides reaches 100%, depending on the type of apparatus and its place in the TPP scheme. Increasing the efficiency of power heat exchange equipment can be achieved primarily by intensifying heat transfer from the steam side of the apparatus.

Heat transfer intensification

One of the ways to intensify heat transfer in HE is associated with the use of differently profiled tubes. According to experts, tubes in which artificial roughness occurs both on the outside and on the inside can find real application in condensing TA. inside. The intensification of heat transfer from the steam side in this case is determined by a change in the hydrodynamics of the condensate film on the profiled surface of the tube - a decrease due to the action of forces surface tension the average thickness of the condensate film, changing the trajectory of its movement and turbulence. Intensification from the water side is also determined by the hydrodynamics of the flow - a violation of the ordered flow of fluid in a viscous sublayer due to its turbulence and swirling. However, it should be taken into account that the use of such tubes leads to an increase in the hydraulic resistance of the HE, which means that it requires research to justify the feasibility of using profiled tubes and to select the optimal parameters for their profiling in relation to specific HE and operating conditions of the STU. An analysis of the state of the issue showed that in order to justify the feasibility of using differently profiled tubes in TA PTU, it is necessary to accumulate and generalize data from bench studies and field tests in order to clarify the methods for calculating devices.

The study of hydrodynamics and heat transfer during steam condensation on variously profiled tubes was carried out on: profiled twisted tubes (PVT), longitudinally profiled tubes (PPT), double profile tubes (TDP) and counter-helical tubes (VVT) .

Experiments have established that the hydrodynamics of a condensate film on a vertical HTP differs significantly from the hydrodynamics of a film on a smooth tube. On the profile tube, the process of shrinking the film into the groove and twisting is observed. With a decrease in the pitch between the grooves S, the angle of deviation of the trajectory of the film from the vertical direction increases and the condensate film is pulled into the grooves due to surface tension forces.

The relative effect of heat transfer intensification during the condensation of a stationary vapor on a vertical HTP depends mainly on the flow regime of the condensate film and the parameters of the tube profiling. Depending on the process parameters and profiling parameters, the heat transfer intensity during steam condensation on a transversely streamlined vertical HTP is up to 2.5 times higher than when stationary steam is condensed on a smooth tube.

It is known that the use of vertical PHEs makes it possible to significantly (up to 3.5 times) increase the heat transfer coefficient from the side of the condensing steam. This is explained by the action of surface tension forces on the condensate film on the profiled curvilinear surface of the tube. On the protrusions of the tube, a more intense condensation of vapor occurs, i.e. heat transfer is actually limited by the thickness of the condensate film flowing down the grooves.

It was proposed to additionally profile the PPT with a screw knurling, similar to the PVT. At the same time, it was assumed that the effect of intensification would be realized as outer surface tube (due to changes in the hydrodynamics of the condensate film), and inside it (due to turbulence of the near-wall coolant layer). It has been experimentally established that PPT makes it possible to increase the level of heat transfer during condensation of water vapor by an average of two times compared to a smooth tube. The heat transfer from the side of the condensing steam to the TDP, depending on the "steam-wall" temperature difference, increases by 1.8-2.2 times compared to the PPT. In this case, in our opinion, two effects are manifested: the helical groove, being filled with condensate from the area of ​​the longitudinal grooves, partially removes it in a downward spiral; at the same time, due to rotations, part of the condensate is discharged from the surface of the tube; the helical extrusion of the metal of the longitudinal protrusions, penetrating into the area of ​​condensate flow in the longitudinal grooves, forms alternating local constrictions in them, which disturbs the "thick" laminar condensate film flowing down the longitudinal grooves. The first effect leads to a decrease in the average thickness of the condensate film, and the second - to its additional turbulence. The sum of these effects causes the intensification of heat transfer from the side of the condensing steam.

One of the promising surfaces for PTU heat exchangers is a tube with a counter helical knurling (VVT). Studies of heat transfer during the condensation of stationary steam have shown that the heat transfer coefficient of HWT is 20-30% higher than that of HWT with similar knurling parameters.

The results of comparative tests of more than 100 different condensing heat exchangers with HTP found that the heat transfer intensification, depending on the parameters of the profiling of the tubes and the mode of water flow in them (with optimally selected parameters of the HTP), ranged from 10 to 80%. The hydraulic resistance of the TA in this case increases by approximately the same amount.

It is known that the organization of the mode of drip condensation of steam is the most promising direction heat transfer intensification during steam condensation. The results of studies on the use of a new water repellent (polyfluoroalkyl disulfide) for tubes made of materials MNZh5-1 and L68 showed that the level of the heat transfer coefficient from the vapor side is three to four times higher than the heat transfer during film condensation. Experiments have established that when air enters the steam (at the moment the unit is turned off), the effect of heat transfer intensification decreases sharply and a mixed steam condensation mode is observed. When the experiment was resumed, the drip condensation mode was restored after 15–20 hours of operation of the installation. After the resumption of drip condensation, the level of heat transfer was restored almost to its original value. This result, which is very important for practice, can be explained taking into account contemporary ideas by dynamics biological systems based on the conducted spectrometric studies of the hydrophobic coating of tubes after a series of experiments on droplet condensation. The drip condensation stimulator used in the experiments has both hydrophobic and hydrophilic fragments in its structure. This increases the number of degrees of freedom of the conformational arrangement of the chain. With a sharp decrease in temperature and turning off the supply of steam to the installation, a more compact conformation is realized with the exposure of the hydrophilic fragment of the molecule. All this leads to the implementation of the film (mixed) condensation mode at the initial moment after the steam is switched on again. Further hydrogen bonds cause self-organization of a monomolecular coating with exposure of only hydrophobic regions of molecules, which ensures the resumption of the droplet condensation mode. Actually observed new type self-organizing monomolecular film, which, depending on external conditions can be in different conformational states. The heat transfer coefficient for drip condensation of steam on a smooth horizontal tube (MNZH5-1) is 1.5-2.0 times higher than for film condensation.

The results of bench tests on the use of a water repellent on HTP (the water repellent was applied to the protrusions of HTP) showed that on vertical HTP, separation and discharge of the flowing condensate film from the surface of the tube in the zones of drip condensation was observed, which, in our opinion, caused a decrease in the amount of flowing condensate on the surface vertical HTP and led to an increase in the level of heat transfer by 15-25%,

The results of semi-industrial tests of an experimental module (56 horizontal tubes, material - MNZh5-1), connected in parallel with the K-300-240 turbine condenser at the Reftinskaya GRES, carried out jointly with NPO TsKTI, showed that the water repellent, when applied once to the heat exchange surface, ensured the maintenance of the drip mode. condensation for more than 4500 hours; at the same time, the heat transfer coefficient increased by 35-70% due to the organization of the drip condensation mode.

The vibration of tubes of heat exchangers is reflected in the nature of the flow of the condensate film and, consequently, in the heat transfer from the condensing steam.

Generalization of the experimental data showed that, depending on the specific steam load and vibration parameters, the heat transfer coefficient during steam condensation on a vibrating horizontal tube can increase or decrease compared to the heat transfer coefficient during steam condensation on a stationary tube.

results pilot study are summarized by dependencies that make it possible to calculate the correction value to the heat transfer coefficient from the steam side for horizontal and vertical HE.

Calculations show that the effect of vibration of tubes of horizontal network heaters on heat transfer from the side of condensing steam at the level of specific steam loads typical for HSG is expressed in an increase in the heat transfer coefficient from the steam side by 1.6 to 6.7%.

Based on the results of bench studies and industrial tests, a number of practical advice to improve the efficiency of heat exchangers of vocational schools:

• - The choice of the most effective parameters for tube profiling must be made on the basis of optimization of the profiling parameters and a feasibility study of the entire PTU.
• - When using longitudinally profiled tubes and double-profile tubes in HE, it can be assumed that heat transfer during steam condensation increases by 40-150%, depending on the density heat flow.
• - When used in heat exchangers profiled tubes in order to increase the reliability of connecting tubes to tube sheets, the ends of the tubes should be provided with smooth ones within 150-200 mm.
• - The use of a new promising water repellent in condensing HE PTUs makes it possible to increase the heat transfer coefficient up to 3 times compared to film vapor condensation. However, over time there is a slight decrease in the heat transfer coefficient.

We believe that the decision on the feasibility of applying any development to improve the efficiency of TA PTU should be made on the basis of a comprehensive feasibility study for the entire power plant. At the same time, any TA must be considered not in isolation, but as an organic element of the vocational school. The basics of such a comprehensive technical and economic methodology for specific TA vocational schools and specific operating conditions at TPPs are presented in the works.

Higher education

(DRTI FGBOU SPO "AGTU")

Direction of training

Installation and technical operation refrigeration units _________

COURSE WORK

KR_______15.02.06 _______.00.00.00.PZ

Calculation of the heat transfer coefficient from the outer wall. For laboratory _stand under given conditions. ______________________________________________

(topic title)

The work was approved for protection 27 » Martha 2017

The work was done by a student of the group 431 ____

__________________ __Fomin V.A. ____

Signature (Surname, first and last name)

supervisor work, .__________ ________

Signature (Surname, first and last name)

Fish 2017

federal agency fisheries

Federal State Budgetary Educational Institution

Higher education

"Astrakhan State Technical University»

Dmitrovsky fishery technological Institute(branch)

federal state budget educational institution middle vocational education"Astrakhan State Technical University"

(DRTI FGBOU SPO "AGTU")

EXERCISE

for execution term paper

student study group ___431 __DRTI FGBOU SPO "AGTU"

__________________Fomin Vladimir Alexandrovich ______________________

(last name, first name, patronymic - in full)

TOPIC OF COURSE WORK

Calculation of the heat transfer coefficient from the outer wall. ____________

For a laboratory stand under specified conditions _______________

INITIAL DATA FOR COURSE WORK

W,

Condenser tube inlet water temperature _____ 21,8 o C,

Refrigerant dew point __ 100 o C,

Mass flow of water through the condenser tube _____ 0,0001 kg/s,

Experimental pipe outer diameter ___ 0,0156 m,

Experimental pipe inner diameter 0,018 m,

Presentation of the course work to the head " 27 » Martha 2017

Protection date " _ » ______________ 2017

Introduction

Condensation- the transition of a substance into a liquid solid state from gaseous. The maximum temperature below which condensation occurs is called the critical temperature.

As the steam passes through the pipe, it gradually condenses and a film of condensate forms on the walls. At the same time, the steam flow rate G "and its speed, due to a decrease in the mass of steam, decrease along the length of the pipe, and the condensate flow rate G increases. With an increase in steam speed, the heat transfer intensity increases. This is due to a decrease in the thickness of the condensate film, which flows faster under the influence of the steam flow. Number molecules leaving a unit surface area of ​​a liquid in one second depends on the temperature of the liquid The number of molecules returning from vapor to liquid depends on the concentration of vapor molecules and on average speed them thermal motion, which is determined by the steam temperature. During condensation in pipes, the steam volume is limited by the walls of the pipe. The pipes can be long enough to condense a large number of pair. There is a directed movement of steam, and the speed of the latter can be very high (up to 100 m/s or more). During condensation in pipes, modes are distinguished complete and partial steam condensation. In the first case, all steam entering the pipe is condensed entirely, and a continuous flow of condensate moves at the outlet of the pipe. With partial condensation, a vapor-liquid mixture flows at the outlet of the pipe.

For the emergence volumetric condensation, the vapor must be supersaturated - its density must exceed the density saturated steam. In this case, the vapor must contain the smallest dust particles (aerosols), which serve as ready-made condensation centers. To convert each kilogram of saturated vapor into a liquid, heat must be removed.

The number of molecules emitted from a unit surface area of ​​a liquid in one second depends on the temperature of the liquid. The number of molecules returning from vapor to liquid depends on the concentration of vapor molecules and on the average rate of their thermal motion, which is determined by the temperature of the vapor. From this it follows that for given substance the concentration of vapor molecules at equilibrium of a liquid and its vapor is determined by their equilibrium temperature. The establishment of dynamic equilibrium between the processes of evaporation and condensation with increasing temperature occurs at higher concentrations of vapor molecules. As the temperature rises, the saturation vapor pressure and its density increase, and the density of the liquid decreases due to thermal expansion. In a hermetically sealed vessel, the liquid cannot boil, because at each temperature value, an equilibrium is established between the liquid and its saturated vapor.

Educational and laboratory stands- this is the necessary material and technical base, demanded by both primary and secondary, and higher educational institutions. This base makes it possible to demonstrate various processes, providing effective educational activities.Laboratory stands are used as visual aid, and also help to better assimilate the subject of study. And helps with research

many thermal installations. The stand also provides maximum
visibility of the scheme under study and the process occurring in it. Stands help in the training of highly qualified personnel, armed modern knowledge, practical skills. Student performance practical work is an an important tool deeper assimilation and study educational material and the acquisition of practical skills.

Evaporation is the process by which a substance moves from liquid state into vapor or gaseous, occurring on the surface of a substance. The evaporation process is the reverse of the condensation process (transition from vapor to liquid). During evaporation, particles (molecules, atoms) fly out (tear off) from the surface of a liquid or solid, while their kinetic energy must be sufficient to do the work necessary to overcome the forces of attraction from other molecules of the liquid.

Evaporation is an endothermic process in which heat is absorbed phase transition- the heat of evaporation spent on overcoming the forces of molecular cohesion in liquid phase and the work of expansion in the transformation of liquid into vapor. The evaporation process depends on the intensity of the thermal motion of the molecules: the faster the molecules move, the faster the evaporation occurs. Same way An important factor is also the surface area of ​​the liquid from which evaporation occurs.

The evaporation rate depends on:

1. liquid surface area.

2. temperature (increases), although it occurs at any temperature and does not require a constant supply of heat. During evaporation, the temperature of the liquid decreases.

3. movement of molecules over the surface of a liquid or gas,

4. kind of substance.

Evaporation can occur not only from the surface, but also in the bulk of the liquid. Liquids always contain tiny gas bubbles. If the saturation vapor pressure of a liquid is equal to or greater than the external pressure (i.e., the pressure of the gas in the bubbles), the liquid will evaporate into the bubbles. The bubbles filled with steam expand and float to the surface. This process is called boiling.

Heat transfer intensification

Intensification- the process and organization of the development of production, in which the most effective means production and expansion of production. The process of converting the consumption of resources, as well as the use of new equipment, can cause an increase in productivity.

The intensification of heat transfer is one of the most important technical tasks, since an increase in the heat transfer coefficient allows, at a given thermal performance and temperatures of heat carriers, to reduce the heat exchange surface, and therefore reduce the weight, size and cost of the heat exchanger.

In many branches of technology, the task of intensifying the heat transfer process and creating highly efficient heat exchangers is very relevant. To intensify heat transfer processes, the following methods are used:

· Prevention of deposits (sludge, salts, corrosive oxides) by systematic flushing, cleaning and special treatment of heat exchange surfaces and preliminary separation from heat carriers of substances and impurities that give deposits;

· Purging of pipe and annulus spaces from inert gases, which drastically reduce heat transfer during vapor condensation;

· Finning of the heat exchange surface, expedient both for increasing the heat transfer coefficient and for reducing the mass of the heat exchanger. The surface of the fins, which is 5-10 times larger than the surface of the carrier tubes, is not subject to unilateral pressure, and therefore the ribs can be made of a thinner material than the walls of the pipes, and thereby achieve a significant reduction in the weight of the apparatus and the consumption of methane.

The intensification of radiative and convective heat transfer of the basic equation of radiative heat transfer shows that an increase in the specific heat load radiation surface can be achieved mainly by increasing the adiabatic combustion temperature. AT lesser degree the efficiency of radiative heat exchange is affected by the temperature of the combustion products at the outlet of the furnace and the coefficient of thermal efficiency of the heating surfaces of screens and screens. An increase in the adiabatic combustion temperature of a given fuel is possible by reducing the excess air coefficient, reducing losses from chemical underburning, and increasing the temperature of the air used for fuel combustion.

The optimal value of the coefficient of excess air and the regulated chemical underburning in the furnace of this design is given in Ch. 3. Intensification of radiative and convective heat transfer and an increase in air temperature is possible within the limits limited by the technical and economic conditions for the distribution of heat absorption in the boiler elements, the reliability of the air heater and mechanical furnaces with layered fuel combustion. The air heating temperatures recommended based on these provisions are given in. The temperature of the combustion products at the outlet of the furnace largely determines the general technical and economic characteristics of the boiler, including the reliability and uninterrupted operation of its operation. When burning solid fuel, the increase in the temperature of the combustion products at the outlet of the furnace is limited by the conditions of slagging of the heating surfaces of the screens and the heating surfaces located behind the furnace. When burning fuel oil and gas, the temperature of the combustion products at the outlet of the furnace is determined by rational distribution heat absorption of radiative and convective heating surfaces. This question and the recommended temperatures of combustion products at the outlet of the furnace during combustion various kinds fuel and furnace designs are discussed in Ch. 4, 6, 8. The coefficient of thermal efficiency can be increased by increasing slope x heating surfaces, in particular, by using double-light screens and screens, as well as by maintaining clean heating surfaces during their systematic cleaning of contaminants by blowing or by mechanical impact on the pipes.

The intensification of radiative and convective heat transfer, as can be seen from the expressions for determining the heat transfer coefficients, is possible by increasing the speed of the coolant, primarily the combustion products, as well as by reducing the diameter of the pipes d or the diameter of the equivalent channel d K . At the same time, the coefficient convective heat transfer increases proportionally to the gas velocity to the power of 0.6-0.8 and inversely proportional to the determining size d to the power of 0.4-0.2, depending on the location of the pipes in relation to the gas flow. Accordingly, the necessary convective elements of the boiler are reduced. However, with an increase in the speed of gases, there is an increase in the aerodynamic resistance of the heating surface, which is proportional to the square of the speed of the gases, and, accordingly, an increase in the consumption of electricity for traction. In this regard, economically feasible limits for increasing the speed of gases arise, which are also limited (when burning solid fuel) by the conditions of wear of the heating surfaces.

More widely used is the second way to increase the efficiency of convective heat transfer (intensification of radiative and convective heat transfer) - reducing the diameter of pipes and equivalent channels. With a decrease in the diameter of the tubes, the aerodynamic drag of the tube bundles at a constant gas velocity even slightly decreases. Reducing the diameter of pipes used for convective heating surfaces is one of the characteristic trends in the development of boiler designs in recent decades.

A consequence of the intensification of heat transfer processes is an increase in the heat transfer coefficient, which, with clean heat exchange surfaces, is determined by the heat transfer coefficients from the side of the heating and heated coolants. In many cases, the physicochemical properties of the heat carriers used differ significantly, their pressure and temperature, and heat transfer coefficients are not the same. Thus, the value of the heat transfer coefficient on the water side α = 2000 ... 7000 W / (m 2 K), on the side of the gas coolant α ≤ 200 W / (m 2 K), for viscous liquidsα \u003d 100 ... 600 W / (m 2 K). It is obvious that heat transfer intensification should be carried out on the side of the coolant, which has a small value of the heat transfer coefficient. With the same order of values ​​of the heat transfer coefficients of heat carriers, heat transfer intensification can be carried out on both sides of heat transfer, but taking into account operational and technical capabilities.

Usually, the intensification of heat transfer is associated with an increase in energy costs to overcome increasing hydraulic resistance. Therefore, one of the main indicators characterizing the expediency of heat transfer intensification in heat exchangers is its energy efficiency. The increase in the intensity of heat transfer should be commensurate with the increase in hydraulic resistance.

The following main methods of heat transfer intensification are used:

construction of rough surfaces and surfaces complex shape, contributing to the turbulence of the flow in the near-wall layer;

the use of turbulent inserts in the channels;

increase in heat exchange surface area by means of fins;

impact on the coolant flow by electric, magnetic and ultrasonic fields;

turbulence of the near-wall layer by organizing fluctuations in the speed of the oncoming flow and its swirling;

mechanical impact on the heat exchange surface by its rotation and vibration;

the use of a granular nozzle both in a stationary and in a pseudo-moving state;

adding solid particles or gas bubbles to the coolant.

The possibility and expediency of using one or another method of intensification for specific conditions are determined by the technical capabilities and efficiency of this method.

One of the most widely used methods for intensifying heat transfer (increasing heat flow) is the finning of the outer surface of pipes, provided that a coolant with a low value of the heat transfer coefficient is directed into the annular space.

Schemes of some devices used to intensify heat transfer in pipes are given in Table. 7.1.

7.1. Schemes of devices used for intensification

heat transfer

ribbing		ribbing
twisted		Pipe with helical smoothly defined protrusions
Continuous screw agitator		Twisted pipe
Annular channel type diffuser-confuser		Alternating smoothly contoured annular protrusions on inner surface smooth pipe

Vane swirlers, intermittent screw swirlers with a different shape of the central body, etc. are used. It should be noted that simultaneously with an increase in the heat transfer coefficient by 30 ... 40%, there is an increase in hydraulic resistance by 1.5-2.5 times. This is explained by the fact that the dissipation of energy during the disintegration of large-scale vortex structures (they arise when the flow swirls) significantly exceeds the generation of turbulence - to feed the weakening vortices, a continuous supply of energy from the outside is needed.

It has been established that under turbulent and transitional flow regimes, it is advisable to intensify turbulent pulsations not in the flow core, but in the near-wall layer, where the turbulent thermal conductivity is low and the heat flux density is maximum, because this layer accounts for 60 ... 70% of the available temperature difference "wall- liquid". How more number R r, the thinner layer it is expedient to influence.

The above recommendations can be implemented by creating in some way, for example, by knurling, alternating smoothly defined annular protrusions on the inner surface of a smooth pipe. For dropping liquids with P r = 2…80 best results were obtained at t sun /d int = 0.25 ... 0.5 and d sun / d int = 0.94 ... 0.98. So, at R e = 10 5, heat transfer increases by 2.0-2.6 times with an increase in hydraulic resistance by 2.7-5.0 times compared with the heat transfer of a smooth pipe. For air, good results were obtained at t sun /d in = 0.5 ... 1.0 and d sun / d in = 0.9 ... 0.92: in the transition region of the flow (R e = 2000 ... 5000) an increase in heat transfer 2.8 ... 3.5 times with an increase in resistance by 2.8-4.5 times (compared to a smooth pipe).

Methods of mechanical influence on the heat exchange surface and influence on the flow of electric, ultrasonic and magnetic fields have not yet been studied enough.

Convective heat transfer

Convection- this is the movement of heat due to the movement of specific macroscopic volumes of liquid or gas. Convection is always accompanied by heat transfer through conduction.

Under convective heat transfer understand the process of heat propagation in a liquid (or gas) from the surface of a solid body or to its surface simultaneously by convection and thermal conductivity. Such a case of heat propagation is also called heat transfer by contact or simply heat transfer.

The transfer of heat by convection is the more intense, the more turbulently the entire mass of the liquid moves and the more vigorously the mixing of its particles is carried out. That. Convection is associated with mechanical heat transfer and strongly depends on the hydrodynamic conditions of the fluid flow.

According to the nature of the occurrence, two types of the nature of the movement of a liquid are distinguished:

1. free fluid movement (i.e. natural convection) - arises due to the difference in densities of heated and cold liquid particles and is determined by physical properties liquid, its volume and temperature differences between heated and cold particles.

2. forced (forced) fluid movement ( forced convection) occurs under the influence of some foreign pathogen, such as a pump, fan. It is determined by the physical properties of the fluid, its speed, the shape and size of the channel in which the movement is carried out.

AT general case Along with forced movement, free movement can also develop simultaneously. Heat transfer processes are inextricably linked with the conditions of fluid motion. As is known, there are two main flow regimes: laminar and turbulent. At laminar flow The flow is calm and rippling. With turbulent - the movement is disordered, vortex. For heat transfer processes, the mode of movement of the working fluid has a very great importance, since it determines the mechanism of heat transfer.

Mechanism of heat transfer by convection

(convective heat transfer)

Let us consider the process of heat transfer by convection and thermal conductivity from the surface of a solid body to the flow of liquid (or gas) washing it, or, conversely, from the flow to solid body, for example, the wall of the heat exchanger.

In the core of the flow, heat transfer is carried out simultaneously by heat conduction and convection. The mechanism of heat transfer in the core of the flow during turbulent motion of the medium is characterized by intense mixing due to turbulent pulsations, which leads to the equalization of temperatures in the core to a certain average value t av (t av1 or t av2). Accordingly, the heat transfer in the core is determined primarily by the nature of the movement of the coolant, but also depends on its thermal properties. As we approach the wall, the intensity of heat transfer decreases. This is explained by the fact that a thermal boundary layer similar to the hydrodynamic boundary layer is formed near the wall. That. as you get closer to the wall greater value acquires thermal conductivity, and in the immediate vicinity of the wall (in a very thin laminar thermal sublayer) heat transfer is carried out only by thermal conductivity.

The thermal boundary sublayer is considered to be the near-wall layer in which the effect of turbulent fluctuations on heat transfer becomes negligible.

It should be distinguished that the intensity of t/recoil is determined mainly by the thermal resistance of the near-wall sublayer, which, in comparison with the thermal resistance of the core, turns out to be decisive.

With turbulent fluid flow, heat transfer occurs much more intensively than with laminar flow. As the turbulence of the flow increases, mixing increases, which leads to a decrease in the thickness of the boundary layer and an increase in the amount of heat transferred.

One of practical tasks in technology is the development of turbulence during the movement of coolants.

The purpose of the development of turbulence in heat exchange equipment is to reduce the thickness of the thermal boundary sublayer, in this case the process is limited only by convection.

The amount of heat transferred by molecular thermal conductivity is determined by the Fourier law:

t is the temperature at the boundary

The heat transferred by convection is determined according to Newton's law or the law of heat transfer:

(2)

The amount of heat transferred by the surface F, which has a temperature t st to the environment with a temperature t cf, is directly proportional to the heat exchange surface and the temperature difference m/y t st and t cf the environment.

Due to turbulent pulsations, temperatures are equalized and can be equated.

Equating (1) and (2) the equation we get:

But the value is difficult to determine.

heat transfer coefficient, [W / m 2 K] - shows how much heat is transferred from 1 m 2 of the wall surface to the liquid at a temperature difference between the wall and the liquid of one degree.

The value characterizes the intensity of heat transfer between the surface of a body, for example, a solid wall and environment(drop liquid or gas).

The heat transfer process is complex process, and the heat transfer coefficient is complex function various quantities characterizing this process.

The heat transfer coefficient depends on the following factors:

Fluid velocity, its density and viscosity, i.e., variables that determine the fluid flow regime;

The thermal properties of the liquid (specific heat capacity C p, thermal conductivity), as well as the coefficient of volume expansion;