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Hagen-Poiseuille law Hagen-Rubens relation, between electrical and optical constants, Hall Taylor, N S, Halogenated hydrocarbons: Handley and Heggs equation for fixed bed pressure drop, Hankinson and Thomson method, for liquid density: Hardening (precipative) of stainless steels, Hardwick, R, Harris, D, Hausen equation for developing laminar flow, Hays, G F Headers in shell-and-tube heat exchangers, Heads, in heat exchangers: Heat and mass transfer: Heat exchanger design, introduction, Heat exchangers: Heat of vaporisation (see Enthalpy of vaporisation), of pure substances Heat pipes: Heat pumping, relation to heat exchanger network design, Heat storage (see Regenerators and thermal energy storage) entropy generation in, Heat transfer: Heat transfer coefficient: Heat transfer media, Heat transfer salt, Heat transfer regimes: Heat of vaporization, Heated cavity reflectometer, Heating media, for reboilers, Heavy water, physical properties of, Heggs, P J, Helical coils of circular cross section: Helical coils of rectangular cross section, Helical inserts, for enhancement of heat transfer in boiling, Helium: Helmholtz reciprocity principle, in radiative heat transfer, Henry, J A R, Henry-Fauske model, for critical two-phase flow, Henry's law, for partial pressure, Heptadecane: Heptadecene: Heptane: 1-Heptanol: 1-Heptene: Herman, K W, Hermes, C L L, Heterogeneous conveyance in horizontal pipes, Heterogeneous nucleation in boiling, Hewitt, G F Hexachloroethane (Refrigerant 116): Hexacyclopentane, superheated vapor properties, Hexadecane: Hexadecene: 1,5-Hexadiene: Hexagonal cells, in free convection, Hexamethylbenzene: Hexane: Hexanoic acid: 1-Hexanol: 1-Hexene: Hexylbenzene: Hexylcyclohexane: Hexylcyclopentane, Hicks equation, for fixed-bed pressure drop, High pressure closures, ASME VIII code guidance for, High-chrome steels, thermal and mechanical properties, High-finned tubes, correlations for single-phase heat transfer in flow over, Hills, P D Hohlraum cavity, Holdup, in liquid-liquid flow, Holland, guide to national practice for mechanical design of heat exchangers, Homogeneous condensation (fog formation), Homogeneous model: Homogeneous nucleation: Honeycombs: Hopkins, D, Horizontal condensers: Horizontal cylinders: Horizontal layers, of fluid, free convection heat transfer in, Horizontal pipes: Horizontal shell-side evaporator, Horizontal surfaces: Horizontal thermosiphon reboilers: Horizontal tube-side evaporator, Horizontal tubes: Hottel's rule, in absorption of radiation by gases, Hsu criterion, for onset of nucleate boiling, Hybrid cooling towers, Hydraulic conveyance: Hydraulic expansion, of tubes into tube sheets in shell-and-tube heat exchangers, Hydraulic turbine, lost work in, Hydraulic resistance, in flow of supercritical fluids, Hydraulically smooth surface, Hydrazine: Hydrocarbons: Hydrodynamic entrance length, in single-phase flow in ducts, Hydrogen: Hydrogen bromide: Hydrogen chloride: Hydrogen cyanide: Hydrogen fluoride: Hydrogen iodide: Hydrogen peroxide: Hydrogen sulfide: Hydrostatic testing of shell-and-tube heat exchangers, Hysteresis:
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Axial Heat Transfer Rate Operational Domain

DOI 10.1615/hedhme.a.000313

3.10.4 Axial heat transfer rate operational domain

Axial heat transfer rate can be limited by a number of factors (Figure 1):

  • Viscous limit: at the low pressures the friction losses in vapor channel can limit the circulating fluid flow rate;
  • Sonic limit: the velocity of vapor cannot exceed the sonic or choking velocities;
  • Cooling limit: the capacity of the heat sink to provide effective cooling of the condenser is often a major heat pipe heat transfer limitation, which depends on number conditions (variant of heat removal from condenser by radiation is shown in Figure 1);
  • Entrainment limit: entrainment of liquid by the vapor must be avoided, or induced shear stress can slow down liquid back flow and also stimulate the transfer of entrained liquid droplets in the vapor to the condenser which can lead to evaporator dry-out;
  • Capillary limit: every wick has certain pumping capacity threshold for specified liquid and heat pipe design (see Section 311);
  • Boiling limit: if the liquid superheat in the evaporator part of the wick exceeds that required for incipient nucleation of the liquid then boiling process can result in the formation of large vapor bubbles in the wick that block liquid circulation.

Figure 1 Operation domain: maximum heat transfer rate as a function of operating temperature. Key: 1, viscous limit; 2, sonic limit; 3, cooling limit; 4, entrainment limit; 5, capillary limit; 6, boiling limit

Figure 1 illustrates diagrammatically how these six factors combine to give the operational domain for a given design of heat pipe. In practice the heat pipe heat transfer capacity in the operational range generally is bounded by the capillary or/and cooling limits (especially for high temperature heat pipes). However, the viscous, sonic and entrainment limits are often encountered during the startup from the liquid or frozen state. For detail analysis and analytical representations of the limits the reader is referred to Faghri (1995), Chi (1976), Brennan and Kroliczek (1979), Reay (2006), Peterson (1994), Silverstein (1992), Ivanovskii et al. (1982).

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