Boiling Phenomena in Heat Transfer

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Questions and Answers

What distinguishes pool boiling from flow boiling?

Pool boiling occurs in the absence of bulk fluid flow, while flow boiling occurs with fluid motion induced by external means.

Define subcooled boiling and saturated boiling.

Subcooled boiling occurs when the liquid's temperature is below the saturation temperature ($T_{sat}$), while saturated boiling occurs when the temperature equals $T_{sat}$.

What occurs during natural convection boiling?

In natural convection boiling, liquid motion and heat transfer are induced by buoyancy effects, with no bubble formation.

Describe the nucleate boiling regime.

<p>In the nucleate boiling regime, bubbles form and rise, with two key phases: bubble formation before reaching the surface and the development of vapor jets.</p> Signup and view all the answers

What characterizes transition boiling?

<p>Transition boiling is marked by a combination of nucleate and film boiling, with unstable vapor film layers acting as thermal insulation.</p> Signup and view all the answers

What happens to heat transfer as the temperature difference increases in transition boiling?

<p>As the temperature difference ($ΔT_e$) increases, the heat transfer rate ($q''$) decreases until reaching the Leidenfrost point.</p> Signup and view all the answers

In which boiling regime do bubbles collapse before reaching the free surface?

<p>In the nucleate boiling regime, bubbles form but collapse before they can reach the free surface.</p> Signup and view all the answers

Explain how buoyancy affects the liquid motion in natural convection boiling.

<p>Bouyancy causes the slightly heated liquid near the surface to rise, contributing to liquid motion and heat transfer.</p> Signup and view all the answers

What is the significance of the constant Ccr in the context of nucleate boiling?

<p>Ccr is a constant that represents the maximum or critical heat flux in nucleate boiling and its value depends on the heater geometry.</p> Signup and view all the answers

How does the film boiling heat flux relate to the thermal conductivity of vapor?

<p>The heat flux in film boiling is influenced by the thermal conductivity of the vapor, which plays a significant role in heat transfer efficiency.</p> Signup and view all the answers

Describe the flow boiling process and its dependence on external forces.

<p>Flow boiling is the process where fluid movement is induced by an external source, such as a pump, enhancing the phase-change process through convection.</p> Signup and view all the answers

What happens during the bubbly flow regime in a heated tube?

<p>In the bubbly flow regime, bubbles start to form on the tube's inner surfaces and are carried into the flow, creating a bubbly appearance in the fluid.</p> Signup and view all the answers

Explain the slug flow regime and its characteristics.

<p>The slug flow regime occurs when bubbles grow in size and coalesce into slugs of vapor that can occupy up to half of the tube volume.</p> Signup and view all the answers

How does the addition of motion affect heat transfer during external flow boiling?

<p>The added motion in external flow boiling increases both the nucleate boiling heat flux and the critical heat flux considerably.</p> Signup and view all the answers

What role does radiation heat transfer play in film boiling at high surface temperatures?

<p>Radiation heat transfer becomes significant during film boiling when the heater surface temperatures are high enough to contribute to heat loss.</p> Signup and view all the answers

Identify the stages of flow boiling in a heated tube.

<p>The stages include subcooled liquid heat transfer, bubbly flow regime formation, followed by slug flow consisting of coalesced vapor bubbles.</p> Signup and view all the answers

What is the primary condition necessary for condensation to occur?

<p>The vapor temperature must be reduced below its saturation temperature, Tsat.</p> Signup and view all the answers

Describe the difference between dropwise and filmwise condensation.

<p>In dropwise condensation, droplets form on the surface, while in filmwise condensation, a continuous liquid film covers the surface.</p> Signup and view all the answers

What influences the flow direction of the liquid film in filmwise condensation on a vertical plate?

<p>The flow direction is influenced by gravity, causing the liquid film to flow downward.</p> Signup and view all the answers

What happens to the temperature of the condensate from the interface to the wall?

<p>The temperature of the condensate decreases from Tsat at the interface to Ts at the wall.</p> Signup and view all the answers

How does the thickness of the liquid film, δ, change in the flow direction during film condensation?

<p>The thickness of the liquid film, δ, increases in the flow direction, x.</p> Signup and view all the answers

How does the Reynolds number influence condensation heat transfer?

<p>The Reynolds number indicates whether the condensate flow is laminar or turbulent, affecting the rate of heat transfer during condensation.</p> Signup and view all the answers

What is the significance of the modified latent heat of vaporization in the condensation process?

<p>The modified latent heat of vaporization reflects the energy released as vapor condenses at a temperature lower than the saturation temperature, influencing heat transfer rates.</p> Signup and view all the answers

Explain the difference between dropwise and film condensation.

<p>Dropwise condensation involves the formation of discrete droplets that roll down a surface, while film condensation forms a continuous liquid film over the surface.</p> Signup and view all the answers

What factors contribute to an increase in heat transfer coefficients in dropwise condensation?

<p>High heat transfer coefficients in dropwise condensation result from unclean or coated surfaces that inhibit wetting and the absence of a resistance film.</p> Signup and view all the answers

In the context of condensation on a vertical plate, what governs the rate of heat transfer?

<p>The rate of heat transfer from vapor to the plate is governed by the heat released during vapor condensation over the vertical distance.</p> Signup and view all the answers

Why is understanding the type of condensation flow important for engineers?

<p>Understanding the type of condensation flow is crucial for optimizing heat exchangers and improving energy efficiency in thermal systems.</p> Signup and view all the answers

What role does surface cleanliness play in condensation processes?

<p>Surface cleanliness affects wetting properties and can significantly impact the transition between film and dropwise condensation.</p> Signup and view all the answers

How does the temperature difference between the surface and vapor influence condensation heat transfer?

<p>A greater temperature difference between the condensing surface and vapor increases the driving force for heat transfer, enhancing condensation rates.</p> Signup and view all the answers

What happens at the Leidenfrost point in terms of heat flux?

<p>At the Leidenfrost point, the heat flux reaches a minimum.</p> Signup and view all the answers

How is film boiling characterized and what effect does it have on heat transfer?

<p>Film boiling is characterized by the formation of a stable vapor film that reduces thermal conductivity, leading to low heat transfer rates.</p> Signup and view all the answers

What factor dominates heat transfer as the temperature difference (ΔTe) increases in film boiling?

<p>As ΔTe increases, radiation heat transfer becomes more dominant.</p> Signup and view all the answers

Why are empirical correlations necessary in predicting boiling heat transfer rates?

<p>Empirical correlations are necessary due to the complex interactions of fluid mechanics and phase-change thermodynamics during boiling.</p> Signup and view all the answers

What role does the nature of nucleation play in the nucleate boiling regime?

<p>The nature of nucleation influences the rate of heat transfer, depending on the number of active nucleation sites and bubble formation rate.</p> Signup and view all the answers

How does the enthalpy of vaporization (hfg) change with pressure during nucleate boiling?

<p>The enthalpy of vaporization decreases with increasing pressure and reaches zero at the critical point.</p> Signup and view all the answers

What is the Rohsenow correlation used for in pool boiling?

<p>The Rohsenow correlation is used to determine the rate of heat transfer per unit area in nucleate pool boiling.</p> Signup and view all the answers

Why is understanding boiling regimes important for heat transfer analysis?

<p>Understanding boiling regimes is important because different regimes require different heat transfer relations for accurate analysis.</p> Signup and view all the answers

What conditions must be met for boiling to occur at a solid-liquid interface?

<p>For boiling to occur, the surface temperature (Ts) must be greater than the saturation temperature (Tsat) of the liquid.</p> Signup and view all the answers

How does boiling heat transfer differ from other forms of convection?

<p>Boiling heat transfer involves phase change and depends on the latent heat of vaporization and surface tension, resulting in higher heat transfer coefficients compared to single-phase convection.</p> Signup and view all the answers

What role does the excess temperature (∆Te) play in boiling heat transfer?

<p>The excess temperature (∆Te) represents the difference between surface and saturation temperatures and acts as the driving force for heat transfer from the surface into the liquid.</p> Signup and view all the answers

Explain the difference between evaporation and boiling in terms of vapor pressure.

<p>Evaporation occurs when the liquid's vapor pressure is less than the vapor pressure at saturation (Pvap &lt; Psat), while boiling happens at the saturation temperature when vapor bubbles can form and move.</p> Signup and view all the answers

What factors influence the boiling heat flux from a solid surface to a fluid?

<p>The boiling heat flux is influenced by the saturation temperature (Tsat), the surface temperature (Ts), the excess temperature (∆Te), and the physical properties of the fluid.</p> Signup and view all the answers

Flashcards

Boiling

A process where a liquid changes into vapor at the interface between a solid surface and the liquid. It occurs when the surface temperature (Ts) is higher than the liquid's saturation temperature (Tsat).

Boiling Heat Flux

The rate of heat transfer from a heated surface to a boiling liquid. Measured in Watts per square meter (W/m²).

Excess Temperature (∆Te)

The temperature difference between the heated surface (Ts) and the saturation temperature of the liquid (Tsat). It drives the boiling process by providing the energy for vaporization.

Saturation Temperature (Tsat)

The temperature at which a liquid changes into vapor at a given pressure. It's a crucial parameter in boiling.

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Vapor Pressure Difference (Psat - Pvap)

The difference between the saturation pressure of the liquid (Psat) and the vapor pressure (Pvap) at a given temperature. It determines the conditions for evaporation.

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Pool Boiling

Boiling where the liquid motion is induced by natural convection and bubble growth detachment.

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Flow Boiling

Boiling under the influence of external forced fluid motion like pumping.

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Subcooled Boiling

When the main bulk of the liquid is below its saturation temperature.

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Saturated Boiling

When the liquid bulk temperature equals the saturation temperature.

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Excess Temperature (ΔTe)

The temperature difference between the heated surface and the liquid's saturation temperature.

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Natural Convection Boiling

Boiling regime dominated by natural convection, no bubbles form.

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Nucleate Boiling

Boiling regime where bubbles form and grow on the surface.

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Film Boiling

Boiling regime where a vapor film forms, insulating the surface.

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Dropwise Condensation

A type of condensation where the vapor forms droplets on a surface instead of a continuous film, like tiny water beads on a cold window pane.

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Filmwise Condensation

Condensation where the vapor forms a continuous liquid film on the surface, like water running down a shower door.

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Film Thickness (δ)

The thickness of the liquid film that forms during filmwise condensation. It increases as the condensate flows down a vertical surface under gravity.

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Condensation

The process of changing a vapor back into a liquid, usually when the vapor's temperature is reduced below its saturation temperature.

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Leidenfrost Point

The point on the boiling curve where the heat flux reaches its minimum value. This occurs when a stable vapor film forms on the heated surface, significantly reducing heat transfer.

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Boiling Heat Flux (q”)

The rate of heat transfer from a heated surface to a boiling liquid, measured in Watts per square meter (W/m²). It indicates how quickly heat is being transferred to the liquid.

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Rohsenow Correlation

The Rohsenow correlation is an empirical equation used to calculate the rate of heat transfer during nucleate boiling. It considers factors like excess temperature, liquid properties, and surface characteristics.

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Nucleate Pool Boiling

A boiling regime where bubbles form and detach from the heating surface, creating a highly turbulent flow. This regime is characterized by high heat transfer rates.

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Critical Heat Flux (CHF)

The maximum heat transfer rate achievable in nucleate boiling before transitioning to film boiling. This is the point where vapor bubbles cover the heating surface, hindering heat transfer.

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Ccr (Critical Heat Flux Constant)

A dimensionless constant that depends on the geometry of the heating surface and influences the critical heat flux in nucleate boiling.

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Subcooled Flow Boiling

A stage in flow boiling where liquid is heated but not yet boiling, and heat transfer occurs primarily through convection.

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Bubbly Flow Boiling

A stage in flow boiling where bubbles start to form and detach from the heated surface, creating a bubbly flow pattern.

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Slug Flow Boiling

A stage in flow boiling where bubbles grow, coalesce, and form vapor slugs that occupy a significant portion of the flow channel.

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Stable Film Boiling

The flow boiling regime where a continuous vapor film forms on the heated surface, leading to a significant decrease in heat transfer compared to nucleate boiling.

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Reynolds Number (Re)

The rate at which a fluid flows over a surface, expressed as the ratio of inertial forces to viscous forces.

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Film Condensation

A type of condensation where a continuous liquid film forms on the surface.

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Hydraulic Diameter (Dh)

The characteristic length used in Reynolds number calculations for film condensation. It represents an effective flow path.

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Latent Heat of Vaporization (Hfg)

The heat released when vapor condenses at the surface.

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Temperature Difference (ΔT)

The temperature difference between the saturation temperature of the vapor and the surface temperature.

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Heat Transfer Coefficient (h)

The heat transfer coefficient for condensation, which accounts for the rate of heat transfer per unit area and temperature difference.

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Rate of Heat Transfer (q)

The rate at which heat is transferred through a liquid film during condensation.

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Study Notes

Heat and Mass Transfer

  • This is a broad topic covering various subjects within heat and mass transfer.
  • It encompasses boiling, condensation, and other heat transfer processes.
  • Various types of boiling exist based on the presence or absence of bulk fluid flow.

Boiling and Condensation

  • Boiling is a liquid-to-vapor phase change.
  • It occurs at the solid-liquid interface when a liquid at its saturation temperature (Tsat) is brought in contact with a solid surface, which is maintained at a temperature (Ts) sufficiently higher than Tsat.
  • Evaporation occurs at the liquid-vapor interface when the vapor pressure (Pvap) is less than the saturation pressure (Psat).
  • Condensation occurs when the temperature of a vapor is reduced below its saturation temperature (Tsat).
  • Condensation can also occur on the free surface of a liquid, or even in a gas, when the temperature of the liquid or the gas is below Tsat.
  • Two primary forms of condensation are dropwise and film wise.

Boiling Regimes and the Boiling Curve

  • Boiling regimes are categorized based on the presence of bulk fluid flow.

  • Pool Boiling: Boiling that occurs in the absence of bulk fluid flow (stationary liquids)

  • Flow Boiling: Boiling that occurs in the presence of bulk fluid motion

  • Four distinct regimes of boiling include: -Natural convection boiling -Nucleate boiling -Transition boiling -Film boiling

  • The boiling curve demonstrates the relationship between excess temperature (ΔT) and heat flux (q).

    • Excess temperature is the difference (ΔT = Ts − Tsat).

  • Boiling and condensation heat transfer coefficients (h) are significantly higher than those encountered in other convection processes involving a single phase.

  • Boiling exhibits the combined effect of convection and pool boiling in external flow.

  • Different flow regimes are encountered in flow boiling within a heated tube, including mist flow, and different types of flow (annular, slug, and bubbly) as quality increases.

Heat Transfer Correlations in Pool Boiling

  • Heat transfer correlations are empirical.
  • These correlations are necessary to predict the rate of heat transfer in various boiling regimes
  • The rate of heat transfer in nucleate pool boiling is determined using the Rohsenow correlation.
  • The maximum (critical) heat flux in nucleate pool boiling is determined using the Kutateladze correlation.
  • Correlations exist to determine the heat transfer rates for both stable film boiling and for drop wise condensation.

Heat Transfer Correlations in Film Boiling

  • The heat flux in film boiling on a horizontal cylinder or sphere of diameter D is given by the Bromley correlation.

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