Thermodynamics, Heat Transfer, And Fluid Flow. V.2. Heat Transfer (776131), страница 8
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A non-regenerative heat exchangerrejects heat to the surroundings.The heat transfer rate for a heat exchanger can be calculated usingthe equation below.Q̇Rev. 0Uo Ao ∆TlmPage 39HT-02BOILING HEAT TRANSFERHeat TransferBOILING HEAT TRANSFERThe formation of steam bubbles along a heat transfer surface has asignificant effect on the overall heat transfer rate.EO 1.15DESCRIBE the process that occurs in the followingregions of the boiling heat transfer curve:a.Nucleate boilingb.Partial film boilingc.Film boilingd.Departure from nucleate boiling (DNB)e.Critical heat fluxBoilingIn a nuclear facility, convective heat transfer is used to remove heat from a heat transfer surface.The liquid used for cooling is usually in a compressed state, (that is, a subcooled fluid) atpressures higher than the normal saturation pressure for the given temperature.
Under certainconditions, some type of boiling (usually nucleate boiling) can take place. It is advisable,therefore, to study the process of boiling as it applies to the nuclear field when discussingconvection heat transfer.More than one type of boiling can take place within a nuclear facility, especially if there is arapid loss of coolant pressure. A discussion of the boiling processes, specifically local and bulkboiling, will help the student understand these processes and provide a clearer picture of whybulk boiling (specifically film boiling) is to be avoided in nuclear facility operations.Nucleate BoilingThe most common type of local boiling encountered in nuclear facilities is nucleate boiling. Innucleate boiling, steam bubbles form at the heat transfer surface and then break away and arecarried into the main stream of the fluid. Such movement enhances heat transfer because the heatgenerated at the surface is carried directly into the fluid stream.
Once in the main fluid stream,the bubbles collapse because the bulk temperature of the fluid is not as high as the heat transfersurface temperature where the bubbles were created. This heat transfer process is sometimesdesirable because the energy created at the heat transfer surface is quickly and efficiently"carried" away.HT-02Page 40Rev. 0Heat TransferBOILING HEAT TRANSFERBulk BoilingAs system temperature increases or system pressure drops, the bulk fluid can reach saturationconditions.
At this point, the bubbles entering the coolant channel will not collapse. The bubbleswill tend to join together and form bigger steam bubbles. This phenomenon is referred to as bulkboiling. Bulk boiling can provide adequate heat transfer provided that the steam bubbles arecarried away from the heat transfer surface and the surface is continually wetted with liquidwater. When this cannot occur film boiling results.Film BoilingWhen the pressure of a system drops or the flow decreases, the bubbles cannot escape as quicklyfrom the heat transfer surface.
Likewise, if the temperature of the heat transfer surface isincreased, more bubbles are created. As the temperature continues to increase, more bubbles areformed than can be efficiently carried away. The bubbles grow and group together, coveringsmall areas of the heat transfer surface with a film of steam. This is known as partial filmboiling. Since steam has a lower convective heat transfer coefficient than water, the steampatches on the heat transfer surface act to insulate the surface making heat transfer more difficult.As the area of the heat transfer surface covered with steam increases, the temperature of thesurface increases dramatically, while the heat flux from the surface decreases.
This unstablesituation continues until the affected surface is covered by a stable blanket of steam, preventingcontact between the heat transfer surface and the liquid in the center of the flow channel. Thecondition after the stable steam blanket has formed is referred to as film boiling.The process of going from nucleate boiling to film boiling is graphically represented in Figure13.
The figure illustrates the effect of boiling on the relationship between the heat flux and thetemperature difference between the heat transfer surface and the fluid passing it.Rev. 0Page 41HT-02BOILING HEAT TRANSFERHeat TransferFigure 13Boiling Heat Transfer CurveFour regions are represented in Figure 13. The first and second regions show that as heat fluxincreases, the temperature difference (surface to fluid) does not change very much.
Better heattransfer occurs during nucleate boiling than during natural convection. As the heat flux increases,the bubbles become numerous enough that partial film boiling (part of the surface beingblanketed with bubbles) occurs. This region is characterized by an increase in temperaturedifference and a decrease in heat flux. The increase in temperature difference thus causes totalfilm boiling, in which steam completely blankets the heat transfer surface.Departure from Nucleate Boiling and Critical Heat FluxIn practice, if the heat flux is increased, the transition from nucleate boiling to film boiling occurssuddenly, and the temperature difference increases rapidly, as shown by the dashed line in thefigure. The point of transition from nucleate boiling to film boiling is called the point ofdeparture from nucleate boiling, commonly written as DNB.
The heat flux associated with DNBis commonly called the critical heat flux (CHF). In many applications, CHF is an importantparameter.HT-02Page 42Rev. 0Heat TransferBOILING HEAT TRANSFERFor example, in a reactor, if the critical heat flux is exceeded and DNB occurs at any locationin the core, the temperature difference required to transfer the heat being produced from thesurface of the fuel rod to the reactor coolant increases greatly.
If, as could be the case, thetemperature increase causes the fuel rod to exceed its design limits, a failure will occur.The amount of heat transfer by convection can only be determined after the local heat transfercoefficient is determined. Such determination must be based on available experimental data.After experimental data has been correlated by dimensional analysis, it is a general practice towrite an equation for the curve that has been drawn through the data and to compareexperimental results with those obtained by analytical means.
In the application of any empiricalequation for forced convection to practical problems, it is important for the student to bear inmind that the predicted values of heat transfer coefficient are not exact. The values of heattransfer coefficients used by students may differ considerably from one student to another,depending on what source "book" the student has used to obtain the information. In turbulentand laminar flow, the accuracy of a heat transfer coefficient predicted from any availableequation or graph may be no better than 30%.SummaryThe important information in this chapter is summarized below.Boiling Heat Transfer Summary•Nucleate boiling is the formation of small bubbles at a heat transfer surface. Thebubbles are swept into the coolant and collapse due to the coolant being asubcooled liquid.
Heat transfer is more efficient than for convection.•Bulk boiling occurs when the bubbles do not collapse due to the coolant beingat saturation conditions.•Film boiling occurs when the heat transfer surface is blanketed with steambubbles and the heat transfer coefficient rapidly decreases.•Departure from nucleate boiling (DNB) occurs at the transition from nucleate tofilm boiling.•Critical heat flux (CHF) is the heat flux that causes DNB to occur.Rev. 0Page 43HT-02HEAT GENERATIONHeat TransferHEAT GENERATIONHeat generation and power output in a reactor are related. Reactorpower is related to the mass flow rate of the coolant and thetemperature difference across the reactor core.EO 2.1DESCRIBE the power generation process in a nuclearreactor core and the factors that affect the powergeneration.EO 2.2DESCRIBE the relationship between temperature, flow,and power during operation of a nuclear reactor.EO 2.3DEFINE the following terms:a.Nuclear enthalpy rise hot channel factorb.Average linear power densityc.Nuclear heat flux hot channel factord.Heat generation rate of a coree.Volumetric thermal source strengthEO 2.4CALCULATE the average linear power density for anaverage reactor core fuel rod.EO 2.5DESCRIBE a typical reactor core axial and radial fluxprofile.EO 2.6DESCRIBE a typical reactor core fuel rod axial andradial temperature profile.Heat GenerationThe heat generation rate in a nuclear core is directly proportional to the fission rate of the fueland the thermal neutron flux present.
On a straight thermodynamic basis, this same heatgeneration is also related to the fluid temperature difference across the core and the mass flowrate of the fluid passing through the core. Thus, the size of the reactor core is dependent uponand limited by how much liquid can be passed through the core to remove the generated thermalenergy. Many other factors affect the amount of heat generated within a reactor core, but itslimiting generation rate is based upon how much energy can safely be carried away by thecoolant.HT-02Page 44Rev. 0Heat TransferHEAT GENERATIONThe fission rate within a nuclear reactor is controlled by several factors.
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