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Hein Auracher - One of the best experts on this subject based on the ideXlab platform.

  • Reconstruction of local heat fluxes in pool Boiling experiments along the entire Boiling Curve from high resolution transient temperature measurements
    International Journal of Heat and Mass Transfer, 2008
    Co-Authors: Yi Heng, Martin Buchholz, Hein Auracher, Adel Mhamdi, Sven Groß, Arnold Reusken, Wolfgang Marquardt
    Abstract:

    In this paper, we consider a transient inverse heat conduction problem (IHCP) defined on an irregular three-dimensional (3D) domain in pool Boiling experiments. Heat input to a circular copper heater of 35 mm diameter and 7 mm thickness is provided by a resistance heating foil pressed to the bottom of the heater. The heat flux at the inaccessible Boiling side is estimated from a number of temperature readings in the heater volume. These temperatures are measured by some high-resolution microthermocouples, which are mounted 3.6 μm below the surface in the test heater. The IHCP is formulated as a mathematical optimization problem and solved by the conjugate gradient (CG) method. The arising partial differential equations (PDEs) are solved using the software package DROPS. A simulation case study is used to validate the performance of the solution approach. Finally, we apply the solution approach to the IHCP in pool Boiling experiments. The procedure enables the reconstruction of local instantaneous heat flux distribution on the heater surface at different locations along the Boiling Curve.

  • identification of unifying heat transfer mechanisms along the entire Boiling Curve
    International Journal of Thermal Sciences, 2006
    Co-Authors: Torsten Luttich, Wolfgang Marquardt, Martin Buchholz, Hein Auracher
    Abstract:

    Abstract The interfacial geometry and the associated heat transfer mechanisms close to and at the Boiling surface are identified rigorously along the entire Boiling Curve from wall temperature and two-phase flow sensor probe data taken during Boiling of isopropanol. It is conjectured, that the liquid–vapor interface of the wetting structure provides the key mechanism for heat removal due to evaporation along the entire Boiling Curve. Local fluctuations of surface temperature superheat and heat flux are inferred by the solution of a two-dimensional inverse heat conduction problem employing wall temperature data. Peak heat fluxes of several MW⋅m −2 are estimated. In a next step, the interfacial geometry close to the surface is identified. It is shown that the size of the dry spots and the nucleation site density are well connected to the vapor fraction and the interfacial area density as well as to the contact angle of the wetting fluid. Further, we show that a suitably designed single four-sensor optical probe could provide the necessary data required to exactly identify the wetting structure using the wetting structure geometry model proposed here. We further find that interfacial area density accounts for the contact angle of the fluid and correlates well with the Boiling heat flux. In conclusion, the results show that essential mechanisms of heat removal along the entire Boiling Curve can be attributed to interfacial presence and evolution. A single measure, namely the interfacial area density, is suspected to provide the essential mechanism of the heat removal in the Boiling process. Finally, the results are discussed with respect to the candidate models for the interfacial geometry and heat transfer mechanisms proposed in the literature. Only, specifically designed and refined Boiling experiments as well as identification methods along the line presented in this paper are needed to further verify this postulate.

  • A study of local heat transfer mechanisms along the entire Boiling Curve by means of microsensors
    International Journal of Thermal Sciences, 2006
    Co-Authors: Martin Buchholz, Torsten Luttich, Hein Auracher, Wolfgang Marquardt
    Abstract:

    Abstract An array of 36 microthermocouples (38 μm diameter) embedded in a horizontal copper heater (distance to the surface 3.6 μm), a micro optical probe (tip diameter ∼1.5 μm) and a microthermocouple probe (tip diameter ∼16 μm), both moveable above the heater surface, are applied to study heat transfer mechanisms along the entire Boiling Curve under steady-state conditions. Test fluids are isopropanol and FC-3284. In nucleate Boiling, very localized and rapid temperature drops are observed indicating high heat fluxes at the bottom of the bubbles. Already before reaching CHF, hot spots occur the size of which increases towards the Leidenfrost point. In the entire transition Boiling regime wetting events are observed, but no ones in film Boiling. In low heat flux nucleate Boiling small vapor superheats exist in the bubbles and strong superheats in the surrounding liquid. This characteristic changes continuously with increasing wall superheat: the liquid surrounding the vapor approaches saturation whereas the vapor becomes more and more superheated. In film Boiling the bubbles leaving the vapor film can reach superheats of 40 K near the surface. The optical probes confirm a liquid rich layer near the surface between nucleate Boiling and high heat flux transition Boiling. The void fraction in this layer increases continuously with the distance to the surface until a maximum value which seems to be linked to the bubble departure diameter.

  • experimental investigation of local processes in pool Boiling along the entire Boiling Curve
    International Journal of Heat and Fluid Flow, 2004
    Co-Authors: Martin Buchholz, Torsten Luttich, Hein Auracher, Wolfgang Marquardt
    Abstract:

    Abstract Understanding of fundamental local processes in Boiling is a key to improved understanding of the Boiling process in general and is therefore also a key for better insights using modeling and simulation studies. Unfortunately, only little experimental data is available with respect to local processes. Furthermore, data for Boiling on technically thick heaters and for all Boiling regimes is even more limited. Towards improved understanding of the Boiling process, miniaturized sensors for measurements of the local Boiling dynamics and spatial dimensions have been developed. For studies of the two-phase flow characteristics above the heater surface a 4-tip optical probe with tip diameters below 1.5 μm has been developed. Steady-state measurements along the entire Boiling Curve have been carried out for distances between heater and probe ranging from 20 mm down to positions as close as 8 μm. Microthermocouples have been developed for measurements of local temperature fluctuations inside the heater very close to the surface. Thirty-six of these thermocouples have been arranged on an 1 × 1 mm sized quadratic grid at the center of the heater. The junctions of the microthermocouples are located 3.6 μm below the heater surface. These thermocouples are very useful to study local temperature fluctuations and also spatial wetting dynamics. Selected measurements along the entire Boiling Curve are presented.

  • Towards a unifying heat transfer correlation for the entire Boiling Curve
    International Journal of Thermal Sciences, 2004
    Co-Authors: Torsten Luttich, Wolfgang Marquardt, Martin Buchholz, Hein Auracher
    Abstract:

    Abstract The mechanistic understanding of Boiling processes is still inadequate. Major physical effects determining the heat transfer in high heat flux nucleate and transition Boiling regions have not yet been captured adequately. Thus, existing design correlations are often valid only for one of the Boiling regimes. In this paper, the wetting structure close to the Boiling surface is identified using the experimental data from an optical probe, obtained during pool Boiling of FC-72 on a horizontal surface, together with a mathematical model for the interfacial geometry based on two-phase flow averaging theory. In the same framework, a unifying correlation to describe the heat flux along the entire Boiling Curve is presented. The suggested correlation is based on the same physical quantities regardless of the Boiling regime; it employs only a single fitting parameter in its most simple form. Alternative correlations are compared to the suggested correlation and their relative merit is assessed by statistical model discrimination techniques. The results suggest that transfer phenomena associated with the interfacial evolution, in particular the volumetric presence of interface close to the heater surface, together with the superheat, play an important role for the overall Boiling heat transfer mechanism.

Torsten Luttich - One of the best experts on this subject based on the ideXlab platform.

  • identification of unifying heat transfer mechanisms along the entire Boiling Curve
    International Journal of Thermal Sciences, 2006
    Co-Authors: Torsten Luttich, Wolfgang Marquardt, Martin Buchholz, Hein Auracher
    Abstract:

    Abstract The interfacial geometry and the associated heat transfer mechanisms close to and at the Boiling surface are identified rigorously along the entire Boiling Curve from wall temperature and two-phase flow sensor probe data taken during Boiling of isopropanol. It is conjectured, that the liquid–vapor interface of the wetting structure provides the key mechanism for heat removal due to evaporation along the entire Boiling Curve. Local fluctuations of surface temperature superheat and heat flux are inferred by the solution of a two-dimensional inverse heat conduction problem employing wall temperature data. Peak heat fluxes of several MW⋅m −2 are estimated. In a next step, the interfacial geometry close to the surface is identified. It is shown that the size of the dry spots and the nucleation site density are well connected to the vapor fraction and the interfacial area density as well as to the contact angle of the wetting fluid. Further, we show that a suitably designed single four-sensor optical probe could provide the necessary data required to exactly identify the wetting structure using the wetting structure geometry model proposed here. We further find that interfacial area density accounts for the contact angle of the fluid and correlates well with the Boiling heat flux. In conclusion, the results show that essential mechanisms of heat removal along the entire Boiling Curve can be attributed to interfacial presence and evolution. A single measure, namely the interfacial area density, is suspected to provide the essential mechanism of the heat removal in the Boiling process. Finally, the results are discussed with respect to the candidate models for the interfacial geometry and heat transfer mechanisms proposed in the literature. Only, specifically designed and refined Boiling experiments as well as identification methods along the line presented in this paper are needed to further verify this postulate.

  • A study of local heat transfer mechanisms along the entire Boiling Curve by means of microsensors
    International Journal of Thermal Sciences, 2006
    Co-Authors: Martin Buchholz, Torsten Luttich, Hein Auracher, Wolfgang Marquardt
    Abstract:

    Abstract An array of 36 microthermocouples (38 μm diameter) embedded in a horizontal copper heater (distance to the surface 3.6 μm), a micro optical probe (tip diameter ∼1.5 μm) and a microthermocouple probe (tip diameter ∼16 μm), both moveable above the heater surface, are applied to study heat transfer mechanisms along the entire Boiling Curve under steady-state conditions. Test fluids are isopropanol and FC-3284. In nucleate Boiling, very localized and rapid temperature drops are observed indicating high heat fluxes at the bottom of the bubbles. Already before reaching CHF, hot spots occur the size of which increases towards the Leidenfrost point. In the entire transition Boiling regime wetting events are observed, but no ones in film Boiling. In low heat flux nucleate Boiling small vapor superheats exist in the bubbles and strong superheats in the surrounding liquid. This characteristic changes continuously with increasing wall superheat: the liquid surrounding the vapor approaches saturation whereas the vapor becomes more and more superheated. In film Boiling the bubbles leaving the vapor film can reach superheats of 40 K near the surface. The optical probes confirm a liquid rich layer near the surface between nucleate Boiling and high heat flux transition Boiling. The void fraction in this layer increases continuously with the distance to the surface until a maximum value which seems to be linked to the bubble departure diameter.

  • experimental investigation of local processes in pool Boiling along the entire Boiling Curve
    International Journal of Heat and Fluid Flow, 2004
    Co-Authors: Martin Buchholz, Torsten Luttich, Hein Auracher, Wolfgang Marquardt
    Abstract:

    Abstract Understanding of fundamental local processes in Boiling is a key to improved understanding of the Boiling process in general and is therefore also a key for better insights using modeling and simulation studies. Unfortunately, only little experimental data is available with respect to local processes. Furthermore, data for Boiling on technically thick heaters and for all Boiling regimes is even more limited. Towards improved understanding of the Boiling process, miniaturized sensors for measurements of the local Boiling dynamics and spatial dimensions have been developed. For studies of the two-phase flow characteristics above the heater surface a 4-tip optical probe with tip diameters below 1.5 μm has been developed. Steady-state measurements along the entire Boiling Curve have been carried out for distances between heater and probe ranging from 20 mm down to positions as close as 8 μm. Microthermocouples have been developed for measurements of local temperature fluctuations inside the heater very close to the surface. Thirty-six of these thermocouples have been arranged on an 1 × 1 mm sized quadratic grid at the center of the heater. The junctions of the microthermocouples are located 3.6 μm below the heater surface. These thermocouples are very useful to study local temperature fluctuations and also spatial wetting dynamics. Selected measurements along the entire Boiling Curve are presented.

  • Towards a unifying heat transfer correlation for the entire Boiling Curve
    International Journal of Thermal Sciences, 2004
    Co-Authors: Torsten Luttich, Wolfgang Marquardt, Martin Buchholz, Hein Auracher
    Abstract:

    Abstract The mechanistic understanding of Boiling processes is still inadequate. Major physical effects determining the heat transfer in high heat flux nucleate and transition Boiling regions have not yet been captured adequately. Thus, existing design correlations are often valid only for one of the Boiling regimes. In this paper, the wetting structure close to the Boiling surface is identified using the experimental data from an optical probe, obtained during pool Boiling of FC-72 on a horizontal surface, together with a mathematical model for the interfacial geometry based on two-phase flow averaging theory. In the same framework, a unifying correlation to describe the heat flux along the entire Boiling Curve is presented. The suggested correlation is based on the same physical quantities regardless of the Boiling regime; it employs only a single fitting parameter in its most simple form. Alternative correlations are compared to the suggested correlation and their relative merit is assessed by statistical model discrimination techniques. The results suggest that transfer phenomena associated with the interfacial evolution, in particular the volumetric presence of interface close to the heater surface, together with the superheat, play an important role for the overall Boiling heat transfer mechanism.

Wolfgang Marquardt - One of the best experts on this subject based on the ideXlab platform.

  • Reconstruction of local heat fluxes in pool Boiling experiments along the entire Boiling Curve from high resolution transient temperature measurements
    International Journal of Heat and Mass Transfer, 2008
    Co-Authors: Yi Heng, Martin Buchholz, Hein Auracher, Adel Mhamdi, Sven Groß, Arnold Reusken, Wolfgang Marquardt
    Abstract:

    In this paper, we consider a transient inverse heat conduction problem (IHCP) defined on an irregular three-dimensional (3D) domain in pool Boiling experiments. Heat input to a circular copper heater of 35 mm diameter and 7 mm thickness is provided by a resistance heating foil pressed to the bottom of the heater. The heat flux at the inaccessible Boiling side is estimated from a number of temperature readings in the heater volume. These temperatures are measured by some high-resolution microthermocouples, which are mounted 3.6 μm below the surface in the test heater. The IHCP is formulated as a mathematical optimization problem and solved by the conjugate gradient (CG) method. The arising partial differential equations (PDEs) are solved using the software package DROPS. A simulation case study is used to validate the performance of the solution approach. Finally, we apply the solution approach to the IHCP in pool Boiling experiments. The procedure enables the reconstruction of local instantaneous heat flux distribution on the heater surface at different locations along the Boiling Curve.

  • identification of unifying heat transfer mechanisms along the entire Boiling Curve
    International Journal of Thermal Sciences, 2006
    Co-Authors: Torsten Luttich, Wolfgang Marquardt, Martin Buchholz, Hein Auracher
    Abstract:

    Abstract The interfacial geometry and the associated heat transfer mechanisms close to and at the Boiling surface are identified rigorously along the entire Boiling Curve from wall temperature and two-phase flow sensor probe data taken during Boiling of isopropanol. It is conjectured, that the liquid–vapor interface of the wetting structure provides the key mechanism for heat removal due to evaporation along the entire Boiling Curve. Local fluctuations of surface temperature superheat and heat flux are inferred by the solution of a two-dimensional inverse heat conduction problem employing wall temperature data. Peak heat fluxes of several MW⋅m −2 are estimated. In a next step, the interfacial geometry close to the surface is identified. It is shown that the size of the dry spots and the nucleation site density are well connected to the vapor fraction and the interfacial area density as well as to the contact angle of the wetting fluid. Further, we show that a suitably designed single four-sensor optical probe could provide the necessary data required to exactly identify the wetting structure using the wetting structure geometry model proposed here. We further find that interfacial area density accounts for the contact angle of the fluid and correlates well with the Boiling heat flux. In conclusion, the results show that essential mechanisms of heat removal along the entire Boiling Curve can be attributed to interfacial presence and evolution. A single measure, namely the interfacial area density, is suspected to provide the essential mechanism of the heat removal in the Boiling process. Finally, the results are discussed with respect to the candidate models for the interfacial geometry and heat transfer mechanisms proposed in the literature. Only, specifically designed and refined Boiling experiments as well as identification methods along the line presented in this paper are needed to further verify this postulate.

  • A study of local heat transfer mechanisms along the entire Boiling Curve by means of microsensors
    International Journal of Thermal Sciences, 2006
    Co-Authors: Martin Buchholz, Torsten Luttich, Hein Auracher, Wolfgang Marquardt
    Abstract:

    Abstract An array of 36 microthermocouples (38 μm diameter) embedded in a horizontal copper heater (distance to the surface 3.6 μm), a micro optical probe (tip diameter ∼1.5 μm) and a microthermocouple probe (tip diameter ∼16 μm), both moveable above the heater surface, are applied to study heat transfer mechanisms along the entire Boiling Curve under steady-state conditions. Test fluids are isopropanol and FC-3284. In nucleate Boiling, very localized and rapid temperature drops are observed indicating high heat fluxes at the bottom of the bubbles. Already before reaching CHF, hot spots occur the size of which increases towards the Leidenfrost point. In the entire transition Boiling regime wetting events are observed, but no ones in film Boiling. In low heat flux nucleate Boiling small vapor superheats exist in the bubbles and strong superheats in the surrounding liquid. This characteristic changes continuously with increasing wall superheat: the liquid surrounding the vapor approaches saturation whereas the vapor becomes more and more superheated. In film Boiling the bubbles leaving the vapor film can reach superheats of 40 K near the surface. The optical probes confirm a liquid rich layer near the surface between nucleate Boiling and high heat flux transition Boiling. The void fraction in this layer increases continuously with the distance to the surface until a maximum value which seems to be linked to the bubble departure diameter.

  • experimental investigation of local processes in pool Boiling along the entire Boiling Curve
    International Journal of Heat and Fluid Flow, 2004
    Co-Authors: Martin Buchholz, Torsten Luttich, Hein Auracher, Wolfgang Marquardt
    Abstract:

    Abstract Understanding of fundamental local processes in Boiling is a key to improved understanding of the Boiling process in general and is therefore also a key for better insights using modeling and simulation studies. Unfortunately, only little experimental data is available with respect to local processes. Furthermore, data for Boiling on technically thick heaters and for all Boiling regimes is even more limited. Towards improved understanding of the Boiling process, miniaturized sensors for measurements of the local Boiling dynamics and spatial dimensions have been developed. For studies of the two-phase flow characteristics above the heater surface a 4-tip optical probe with tip diameters below 1.5 μm has been developed. Steady-state measurements along the entire Boiling Curve have been carried out for distances between heater and probe ranging from 20 mm down to positions as close as 8 μm. Microthermocouples have been developed for measurements of local temperature fluctuations inside the heater very close to the surface. Thirty-six of these thermocouples have been arranged on an 1 × 1 mm sized quadratic grid at the center of the heater. The junctions of the microthermocouples are located 3.6 μm below the heater surface. These thermocouples are very useful to study local temperature fluctuations and also spatial wetting dynamics. Selected measurements along the entire Boiling Curve are presented.

  • Towards a unifying heat transfer correlation for the entire Boiling Curve
    International Journal of Thermal Sciences, 2004
    Co-Authors: Torsten Luttich, Wolfgang Marquardt, Martin Buchholz, Hein Auracher
    Abstract:

    Abstract The mechanistic understanding of Boiling processes is still inadequate. Major physical effects determining the heat transfer in high heat flux nucleate and transition Boiling regions have not yet been captured adequately. Thus, existing design correlations are often valid only for one of the Boiling regimes. In this paper, the wetting structure close to the Boiling surface is identified using the experimental data from an optical probe, obtained during pool Boiling of FC-72 on a horizontal surface, together with a mathematical model for the interfacial geometry based on two-phase flow averaging theory. In the same framework, a unifying correlation to describe the heat flux along the entire Boiling Curve is presented. The suggested correlation is based on the same physical quantities regardless of the Boiling regime; it employs only a single fitting parameter in its most simple form. Alternative correlations are compared to the suggested correlation and their relative merit is assessed by statistical model discrimination techniques. The results suggest that transfer phenomena associated with the interfacial evolution, in particular the volumetric presence of interface close to the heater surface, together with the superheat, play an important role for the overall Boiling heat transfer mechanism.

Martin Buchholz - One of the best experts on this subject based on the ideXlab platform.

  • Reconstruction of local heat fluxes in pool Boiling experiments along the entire Boiling Curve from high resolution transient temperature measurements
    International Journal of Heat and Mass Transfer, 2008
    Co-Authors: Yi Heng, Martin Buchholz, Hein Auracher, Adel Mhamdi, Sven Groß, Arnold Reusken, Wolfgang Marquardt
    Abstract:

    In this paper, we consider a transient inverse heat conduction problem (IHCP) defined on an irregular three-dimensional (3D) domain in pool Boiling experiments. Heat input to a circular copper heater of 35 mm diameter and 7 mm thickness is provided by a resistance heating foil pressed to the bottom of the heater. The heat flux at the inaccessible Boiling side is estimated from a number of temperature readings in the heater volume. These temperatures are measured by some high-resolution microthermocouples, which are mounted 3.6 μm below the surface in the test heater. The IHCP is formulated as a mathematical optimization problem and solved by the conjugate gradient (CG) method. The arising partial differential equations (PDEs) are solved using the software package DROPS. A simulation case study is used to validate the performance of the solution approach. Finally, we apply the solution approach to the IHCP in pool Boiling experiments. The procedure enables the reconstruction of local instantaneous heat flux distribution on the heater surface at different locations along the Boiling Curve.

  • identification of unifying heat transfer mechanisms along the entire Boiling Curve
    International Journal of Thermal Sciences, 2006
    Co-Authors: Torsten Luttich, Wolfgang Marquardt, Martin Buchholz, Hein Auracher
    Abstract:

    Abstract The interfacial geometry and the associated heat transfer mechanisms close to and at the Boiling surface are identified rigorously along the entire Boiling Curve from wall temperature and two-phase flow sensor probe data taken during Boiling of isopropanol. It is conjectured, that the liquid–vapor interface of the wetting structure provides the key mechanism for heat removal due to evaporation along the entire Boiling Curve. Local fluctuations of surface temperature superheat and heat flux are inferred by the solution of a two-dimensional inverse heat conduction problem employing wall temperature data. Peak heat fluxes of several MW⋅m −2 are estimated. In a next step, the interfacial geometry close to the surface is identified. It is shown that the size of the dry spots and the nucleation site density are well connected to the vapor fraction and the interfacial area density as well as to the contact angle of the wetting fluid. Further, we show that a suitably designed single four-sensor optical probe could provide the necessary data required to exactly identify the wetting structure using the wetting structure geometry model proposed here. We further find that interfacial area density accounts for the contact angle of the fluid and correlates well with the Boiling heat flux. In conclusion, the results show that essential mechanisms of heat removal along the entire Boiling Curve can be attributed to interfacial presence and evolution. A single measure, namely the interfacial area density, is suspected to provide the essential mechanism of the heat removal in the Boiling process. Finally, the results are discussed with respect to the candidate models for the interfacial geometry and heat transfer mechanisms proposed in the literature. Only, specifically designed and refined Boiling experiments as well as identification methods along the line presented in this paper are needed to further verify this postulate.

  • A study of local heat transfer mechanisms along the entire Boiling Curve by means of microsensors
    International Journal of Thermal Sciences, 2006
    Co-Authors: Martin Buchholz, Torsten Luttich, Hein Auracher, Wolfgang Marquardt
    Abstract:

    Abstract An array of 36 microthermocouples (38 μm diameter) embedded in a horizontal copper heater (distance to the surface 3.6 μm), a micro optical probe (tip diameter ∼1.5 μm) and a microthermocouple probe (tip diameter ∼16 μm), both moveable above the heater surface, are applied to study heat transfer mechanisms along the entire Boiling Curve under steady-state conditions. Test fluids are isopropanol and FC-3284. In nucleate Boiling, very localized and rapid temperature drops are observed indicating high heat fluxes at the bottom of the bubbles. Already before reaching CHF, hot spots occur the size of which increases towards the Leidenfrost point. In the entire transition Boiling regime wetting events are observed, but no ones in film Boiling. In low heat flux nucleate Boiling small vapor superheats exist in the bubbles and strong superheats in the surrounding liquid. This characteristic changes continuously with increasing wall superheat: the liquid surrounding the vapor approaches saturation whereas the vapor becomes more and more superheated. In film Boiling the bubbles leaving the vapor film can reach superheats of 40 K near the surface. The optical probes confirm a liquid rich layer near the surface between nucleate Boiling and high heat flux transition Boiling. The void fraction in this layer increases continuously with the distance to the surface until a maximum value which seems to be linked to the bubble departure diameter.

  • experimental investigation of local processes in pool Boiling along the entire Boiling Curve
    International Journal of Heat and Fluid Flow, 2004
    Co-Authors: Martin Buchholz, Torsten Luttich, Hein Auracher, Wolfgang Marquardt
    Abstract:

    Abstract Understanding of fundamental local processes in Boiling is a key to improved understanding of the Boiling process in general and is therefore also a key for better insights using modeling and simulation studies. Unfortunately, only little experimental data is available with respect to local processes. Furthermore, data for Boiling on technically thick heaters and for all Boiling regimes is even more limited. Towards improved understanding of the Boiling process, miniaturized sensors for measurements of the local Boiling dynamics and spatial dimensions have been developed. For studies of the two-phase flow characteristics above the heater surface a 4-tip optical probe with tip diameters below 1.5 μm has been developed. Steady-state measurements along the entire Boiling Curve have been carried out for distances between heater and probe ranging from 20 mm down to positions as close as 8 μm. Microthermocouples have been developed for measurements of local temperature fluctuations inside the heater very close to the surface. Thirty-six of these thermocouples have been arranged on an 1 × 1 mm sized quadratic grid at the center of the heater. The junctions of the microthermocouples are located 3.6 μm below the heater surface. These thermocouples are very useful to study local temperature fluctuations and also spatial wetting dynamics. Selected measurements along the entire Boiling Curve are presented.

  • Towards a unifying heat transfer correlation for the entire Boiling Curve
    International Journal of Thermal Sciences, 2004
    Co-Authors: Torsten Luttich, Wolfgang Marquardt, Martin Buchholz, Hein Auracher
    Abstract:

    Abstract The mechanistic understanding of Boiling processes is still inadequate. Major physical effects determining the heat transfer in high heat flux nucleate and transition Boiling regions have not yet been captured adequately. Thus, existing design correlations are often valid only for one of the Boiling regimes. In this paper, the wetting structure close to the Boiling surface is identified using the experimental data from an optical probe, obtained during pool Boiling of FC-72 on a horizontal surface, together with a mathematical model for the interfacial geometry based on two-phase flow averaging theory. In the same framework, a unifying correlation to describe the heat flux along the entire Boiling Curve is presented. The suggested correlation is based on the same physical quantities regardless of the Boiling regime; it employs only a single fitting parameter in its most simple form. Alternative correlations are compared to the suggested correlation and their relative merit is assessed by statistical model discrimination techniques. The results suggest that transfer phenomena associated with the interfacial evolution, in particular the volumetric presence of interface close to the heater surface, together with the superheat, play an important role for the overall Boiling heat transfer mechanism.

R H Hong - One of the best experts on this subject based on the ideXlab platform.