- Email: [email protected]
Bulk-micromachined circular foil type micro heat-flux sensor
Sensors and Actuators A 132 (2006) 581–586
Bulk-micromachined circular foil type micro heat-flux sensor S.H. Oh a , S.H. Lee b , J.C. Jeon b , M.H. Kim b , S.S. Lee c,∗ a LG-Philips LCD, 373-1 Guseong-dong Yuseong-gu, Daejeon, Korea Department of Mechanical Engineering, Postech, 373-1 Guseong-dong Yuseong-gu, Daejeon, Korea c Department of Mechanical Engineering, KAIST, 373-1 Guseong-dong Yuseong-gu, Daejeon, Korea
b
Received 6 June 2002; received in revised form 20 November 2002; accepted 5 February 2003 Available online 23 May 2006
Abstract A micro heat-flux sensor with high sensitivity under conditions of low heat flux has been designed, bulk-micromachined and tested in a convective environment. The sensor, which is based on the circular foil type heat-flux sensor, is composed of thermal paths and a thermopile. Thermal path layers of electroplated copper on both sides of a wafer are connected through a bulk-micromachined window. A thermopile consisting of a series of n thermocouples is used to get an n-fold output compared to a single couple. When the sensor is placed on a high temperature wall, heat flux from the wall flows through thermal paths and drains out to the environment, producing a temperature difference along these paths. The heat flux is obtained by measuring the temperature difference using a thermopile of Ni–Cr thermocouples. The calibrated sensitivity of the micro heat-flux sensor is 0.17–1.90 V/(mW cm−2 ) in the heat flux range 0–180 mW/cm2 . © 2005 Elsevier B.V. All rights reserved. Keywords: Heat flux; Bulk-micromachining; Copper electroplating; Circular foil gauge; Convective environment
1. Introduction Heat flux (heat transfer per unit area) is one of the most important boundary conditions in heat transfer problems along with temperature. The heat flux can be calibrated by measuring both the heat transfer coefficient and the temperature of the wall surface. However, in real situations, it is very difficult to simultaneously obtain exact values for both of these quantities. Much research has, therefore, been directed towards developing heat-flux sensors capable of directly measuring the heat flux without the wall temperature or heat transfer coefficient. Heat-flux sensors are classified into three categories according to their measurement principles: the gradient method, the transient method and the balanced method [1]. Among them, the gradient method, the most frequently used of these methods, is based on Fourier’s conduction law, which states that the heat flux is proportional to the thermal conductivity and the temperature gradient. Gradient heat-flux ∗
Corresponding author. Tel.: +82 42 869 3046; fax: +82 42 869 3210. E-mail address: [email protected] (S.S. Lee).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2003.02.001
sensors obtain heat flux information by measuring the temperature difference across a thermal resistance and are of two types, according to the heat flow direction: the layered type sensor and the circular foil type sensor. Fig. 1 shows a schematic representation of the principle of the layered type sensor in which heat flow is perpendicular to the thermal resistance layer. A thicker thermal resistance layer produces a larger temperature difference, T = TA − TB , and more accurate results. However, a thinner resistance layer gives faster response. Fast heat transfer phenomena such as shock waves need a fast response heatflux sensor; the pursuit of fast response has led to the use of thinner thermal resistance layers [2–5]. However, since the reduction of the thickness of the thermal resistance layer causes a reduction in the temperature difference, a thermopile composed of a series of thermocouples is used to overcome the reduction in output signal. In this study, we present a novel heat-flux sensor that has high sensitivity under conditions of low heat flux. The sensor is based on the circular foil type sensor suggested in 1953 by Gordon for the measurement of radiative heat transfer and later developed for radiative as well as conductive cases [6–8].
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S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
Fig. 1. Principle of the layered type heat-flux sensor.
Fig. 2 shows the principle of the circular foil type sensor in radiative and conductive heat-flux situations. The circular foil type sensor receives heat flux through the constantan disk. If the backside of the disk is properly insulated, the received heat flows to the edges of the disk and drains out to a copper block which acts as a heat sink. This produces a temperature difference between the center (A or A ) and edges (B or B ) of the disk. The heat flux is obtained by calibrating the temperature difference.
2. Device and fabrication Fig. 3 shows the principle of the bulk-micromachined micro heat-flux sensor that measures heat flux from the wall. The heat flux from the wall is transferred to the sensor through the contact surface and goes out to the environment along the thermal path, A → B → C → D → E, producing a temperature difference along its path. The heat flux from the wall can be obtained by measuring and calibrating the temperature difference between points C and D. Fig. 4 shows the crosssection and top view of the sensor. Thermal path layers of copper (Cu) electroplated on both sides of the wafer are connected through a bulk-micromachined window. To enhance sensor sensitivity in conditions of low heat flux, the thermal path is placed in the vertical direction (perpendicular to the thermal resistance layer, A → B → C in Fig. 3) as well as in the lateral direction (along the thermal resistance layer, C → D in Fig. 3). This change of design enables extension of
Fig. 2. Principle of the circular foil type heat-flux sensor in (a) radiative and (b) conductive heat-flux situations.
the distance between two thermocouple junctions C and D , which effectively increases the thickness of the thermal resistance layer. Fig. 5 shows the principle of the petal shape of the thermopile. The temperatures at points 1, 2, . . . N are the same and so are the temperatures at points 1 , 2 , . . . N . The output signal corresponding to the temperature difference between points 1 and 1 (or 2 and 2 , . . ., or N and N ) is amplified N times using the thermopile of 2N thermocouple junctions.
Fig. 3. Principle of the bulk-micromachined micro heat-flux sensor.
S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
Fig. 4. Schematic view of: (a) cross-section and (b) top view of the micro heat-flux sensor.
Fig. 6 shows the fabrication process of the sensor. The fabrication starts with a bare 3 in. 100-oriented 400 m thick silicon wafer. A 0.6 m thick thermal silicon oxide layer is grown in wet atmospheric conditions. After backside patterning and oxide etching, bulk-micromachining in 25 wt% TMAH solution at 90 ◦ C for 7 h and 10:1 BHF solution in 1 min is used to form a opening window of dimension
Fig. 5. Principle of the petal shape of the thermopile.
Fig. 6. Fabrication process for the heat-flux sensor.
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micromachined window and the thermocouple junctions. The wrinkled membrane of the window is caused by residual stress in the electroplated Cu layer.
3. Test and calibration
Fig. 7. Pictures of the fabricated micro heat-flux sensor: (a) completed heatflux sensor and (b) opening window and the thermocouple junctions.
250 m × 250 m on the front side. Another 1 m thick layer of thermal silicon oxide is grown to be a thermal insulation layer for preventing heat flow through the silicon substrate. After thermal evaporation of 0.01 m thick layer of Cr and 0.1 m thick layer of Au on both sides of the wafer, 10 m thick layer of Cu is electroplated on both sides of the wafer to form thermal paths from the backside to the front side of the wafer. The Cr and Au layers function as an adhesion layer for Au evaporation and a seed layer for Cu electroplating, respectively. A 0.2 m thick electric insulation layer of spinon-glass (SOG) is spin-coated and cured. Two 0.05 m thick thermocouple metals, Ni and Cr, are thermally evaporated and patterned to create the petal-shaped thermopile using a lift-off process. Finally, another 0.2 m thick insulation layer of SOG is spin-coated, cured and patterned to provide wire bonding electrodes and thermal paths to the environment. Fig. 7(a) shows pictures of the completed heat-flux sensor. Fig. 7(b) shows an expanded picture of the bulk-
In this study, we chose a calibration method using a convection facility since heat transfer in a convective environment is the most frequently encountered situation. Fig. 8 shows a schematic view of the experimental setup of the small wind tunnel. In a convective environment, the most important parameter is the heat transfer coefficient at the surface where heat transfer occurs. Even though the same heat flux occurs, the output signal of the sensor may change with the heat transfer coefficient because of the change in the thermal resistance. Accounting for the effect of the heat transfer coefficient is essential in measuring heat flux in a convective environment. In the case of forced convection, the Nusselt number, c Nu = hL k , a non-dimensional form of the heat transfer coefc ficient, is a function of the Reynolds number, Re = ρVL μ and the Prandtl number, Pr = αν , where h is the heat transfer coefficient, Lc the characteristic length, k the fluid conductivity, ν the kinematic viscosity, α the fluid thermal diffusivity, ρ the fluid density, V the free stream velocity and μ is the dynamic viscosity. Since the Prandtl number is regarded as constant in air, the Nusselt number can be represented as a function of the Reynolds number. Thus, in our tests, the heat-flux sensor is calibrated with the Reynolds number. The design of the wind tunnel used to produce the convective environment is adopted from Holemberg et al. [9]. Their report documents the National Institute of Standards and Technology (NIST) standard convective facility in which heat-flux sensors are calibrated. The flow of the wind tunnel is generated by a blower with an ac motor and a variable frequency driver. It flows through a flexible hose, a first diffuser, a heat exchanger, a second diffuser, a nozzle, a settling chamber, a contraction chamber and a test section, as shown in Fig. 8.
Fig. 8. Schematic view of the small wind tunnel.
S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
The flexible hose isolates the wind tunnel from blower vibration. The first diffuser is 899 mm long unit and expands the flow to the final section which has dimensions 260 mm × 260 mm. The heat exchanger is an aluminum, liquid-to-air and fin-tube type. A constant temperature bath is connected to a heat exchanger to maintain the bulk flow at constant temperature. The second diffuser is a 200 mm long unit and expands flow to a final cross-section of 300 mm × 300 mm. The shape of the aluminum nozzle is adopted from the American Society of Mechanical Engineers (ASME) standards for orifices and nozzles. By measuring the difference in pressure before and after the nozzle, the volumetric flow rate is evaluated. The flow from the nozzle goes into the 1200 mm long settling chamber. A screen is inserted between the settling chamber and the contraction chamber. To reduce turbulence intensity, a 2-D contraction of contraction ratio 30:1 is used. The design of the curved end is based on the results of Morel [10]. After the contraction chamber, the flow finally arrives in the test section where the heat-flux sensor is placed. Fig. 9 shows pictures of the test section. The test section is a 20 mm thick copper block under which a heater is placed. The sensor is placed on this copper block, which is used to transform the non-uniform heat flux from the heater into a uniform heat flux over the sensing surface. Thermal insulation is used to reduce backward heat loss.
Fig. 9. Pictures of the test section: (a) sensor on the copper block of the test section and (b) outside of the test section.
585
Fig. 10. Result of the calibration of the heat-flux sensor with the Reynolds number, Re.
4. Results and discussions Fig. 10 shows the calibration result from measurements of the output voltage of the heat-flux sensor with changing heat flux at various values of the Reynolds number. For a given Reynolds number, the output voltage of the sensor increases as the heat flux increases. For a given heat flux, the output voltage is fairly constant for Reynolds numbers of 4000–8000 and decreases after Reynolds number of 8000, except for the high heat flux condition (HF = 180 mW/cm2 ), under which the output voltage decreases as the Reynolds number increases in the range of Reynolds number of 4000–12,000. The latter phenomenon seems to be due to the difference in thermal resistance between the part of the surface of the copper block where the sensor is placed and the part of the surface with no sensor. Since the sensor itself provides thermal resistance in heat transfer situations, heat-flux lines are distorted near the sensor. At higher Reynolds numbers, the heat transfer coefficient increases and the thermal resistance on the surface of the copper block decreases, without changing the thermal resistance of the sensor. This effect causes a relatively lower amount of heat flux to drain out to the environment through the sensor and, therefore, decreases the output voltage of the sensor. When the Reynolds number is 0, the output voltage is quite low, since it is difficult for the thermal energy in the copper block to drain out to the environment. The calibrated sensitivity of the heat-flux sensor is in the range 0.17–1.90 V/(mW cm−2 ) in the heat flux range 0–180 mW/cm2 . These values are about 20 times higher than those from Hager’s heat-flux sensor [5]. However, our sensor does not display good linearity.
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S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
a series of thermocouple, of electroplated Ni–Cr has the petal shape to amplify the output signal of the sensor. For the calibration of the sensor, a small wind tunnel was designed and fabricated under the NIST standard for a convective environment. The calibrated sensitivity of the sensor is 0.17–1.90 V/(mW cm−2 ) in the heat flux range 0–180 mW/cm2 . For a given Reynolds number, the output voltage increases as the heat flux from the wall increases. For a given heat flux, the output voltage is fairly constant for Reynolds numbers of 4000–8000, except for the high heat flux condition HF = 180, under which the output voltage decreases as the Reynolds number increases.
Acknowledgement This work was supported by the Brain Korea 21 Project.
Fig. 11. Effect of the Reynolds number on the output voltage of the heat-flux sensor.
Fig. 11 shows the effect of the Reynolds number on the output voltage of the heat-flux sensor. For a given heat flux condition, the output voltage is fairly constant for Reynolds numbers of 4000–8000, except for the high heat flux condition HF = 180, under which the output voltage decreases with increasing Reynolds. The decrease of output voltage at high Reynolds number and the high heat flux conditions seems to be due to the thermal resistance of the heat-flux sensor. The higher the Reynolds number and the heat flux, the bigger the effect of the thermal resistance of the heat-flux sensor.
5. Conclusions A micro heat-flux sensor with high sensitivity under conditions of low heat flux has been designed, bulk-micromachined and tested in a convective environment. The sensor is based on the circular foil type heat-flux gauge, which is composed of thermal paths and a thermopile. Thermal paths of electroplated Cu on the both sides of a silicon wafer are connected through a bulk-micromachined window and the thermopile,
References [1] T.E. Diller, Advances in heat flux measurements, Adv. Heat Transfer 23 (1983) 279–368. [2] M. Hayashi, A. Sakrai, S. Aso, A study of a multi-layered thin film heat transfer gauge and a new method of measuring heat transfer rate with it, Jpn. Soc. Aeronaut. Space Sci. Trans. 30 (1987) 91– 101. [3] A.H. Epstein, G.R. Guenette, R.J.G. Norton, Y. Gao, High-frequency response heat-flux gauge, Rev. Sci. Instrum. 57 (1985) 639–649. [4] J.M. Hager, S. Simmons, D. Smith, S. Onishi, L.W. Langley, T.E. Diller, Experimental performance of a heat flux microsensor, ASME J. Eng. Gas Turbines Power 113 (1991) 246–250. [5] J.M. Hager, S. Onishi, L.W. Langley, T.E. Diller, Heat flux microsensors, in: R.K. Shah (Ed.), Heat Transfer Measurements, Analysis and Flow Visualization, ASME, New York, 1989, pp. 1–8. [6] R. Gordon, An instrument for the direct measurement of intense thermal radiation, Rev. Sci. Instrum. 24 (5) (1953) 366–370. [7] R. Gordon, A transducer for the measurement of heat flow rate, J. Heat Transfer l82 (1960) 396–398. [8] C.H. Kuo, A.K. Kulkarni, Analysis of heat flux measurement by circular foil gauges in a mixed convection/radiation environment, J. Heat Transfer 113 (1991) 1037–1040. [9] D. Holemberg, K. Steckler, C. Womeldorf, W. Grosshandler, Facility for calibrating heat flux sensors in a convective environment, Proc. ASME Heat Transfer Div. 3 (1997) 165–171. [10] T. Morel, Design of two-dimensional wind tunnel contractions, J. Fluid Eng. (1997) 371–377.
Bulk-micromachined circular foil type micro heat-flux sensor S.H. Oh a , S.H. Lee b , J.C. Jeon b , M.H. Kim b , S.S. Lee c,∗ a LG-Philips LCD, 373-1 Guseong-dong Yuseong-gu, Daejeon, Korea Department of Mechanical Engineering, Postech, 373-1 Guseong-dong Yuseong-gu, Daejeon, Korea c Department of Mechanical Engineering, KAIST, 373-1 Guseong-dong Yuseong-gu, Daejeon, Korea
b
Received 6 June 2002; received in revised form 20 November 2002; accepted 5 February 2003 Available online 23 May 2006
Abstract A micro heat-flux sensor with high sensitivity under conditions of low heat flux has been designed, bulk-micromachined and tested in a convective environment. The sensor, which is based on the circular foil type heat-flux sensor, is composed of thermal paths and a thermopile. Thermal path layers of electroplated copper on both sides of a wafer are connected through a bulk-micromachined window. A thermopile consisting of a series of n thermocouples is used to get an n-fold output compared to a single couple. When the sensor is placed on a high temperature wall, heat flux from the wall flows through thermal paths and drains out to the environment, producing a temperature difference along these paths. The heat flux is obtained by measuring the temperature difference using a thermopile of Ni–Cr thermocouples. The calibrated sensitivity of the micro heat-flux sensor is 0.17–1.90 V/(mW cm−2 ) in the heat flux range 0–180 mW/cm2 . © 2005 Elsevier B.V. All rights reserved. Keywords: Heat flux; Bulk-micromachining; Copper electroplating; Circular foil gauge; Convective environment
1. Introduction Heat flux (heat transfer per unit area) is one of the most important boundary conditions in heat transfer problems along with temperature. The heat flux can be calibrated by measuring both the heat transfer coefficient and the temperature of the wall surface. However, in real situations, it is very difficult to simultaneously obtain exact values for both of these quantities. Much research has, therefore, been directed towards developing heat-flux sensors capable of directly measuring the heat flux without the wall temperature or heat transfer coefficient. Heat-flux sensors are classified into three categories according to their measurement principles: the gradient method, the transient method and the balanced method [1]. Among them, the gradient method, the most frequently used of these methods, is based on Fourier’s conduction law, which states that the heat flux is proportional to the thermal conductivity and the temperature gradient. Gradient heat-flux ∗
Corresponding author. Tel.: +82 42 869 3046; fax: +82 42 869 3210. E-mail address: [email protected] (S.S. Lee).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2003.02.001
sensors obtain heat flux information by measuring the temperature difference across a thermal resistance and are of two types, according to the heat flow direction: the layered type sensor and the circular foil type sensor. Fig. 1 shows a schematic representation of the principle of the layered type sensor in which heat flow is perpendicular to the thermal resistance layer. A thicker thermal resistance layer produces a larger temperature difference, T = TA − TB , and more accurate results. However, a thinner resistance layer gives faster response. Fast heat transfer phenomena such as shock waves need a fast response heatflux sensor; the pursuit of fast response has led to the use of thinner thermal resistance layers [2–5]. However, since the reduction of the thickness of the thermal resistance layer causes a reduction in the temperature difference, a thermopile composed of a series of thermocouples is used to overcome the reduction in output signal. In this study, we present a novel heat-flux sensor that has high sensitivity under conditions of low heat flux. The sensor is based on the circular foil type sensor suggested in 1953 by Gordon for the measurement of radiative heat transfer and later developed for radiative as well as conductive cases [6–8].
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S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
Fig. 1. Principle of the layered type heat-flux sensor.
Fig. 2 shows the principle of the circular foil type sensor in radiative and conductive heat-flux situations. The circular foil type sensor receives heat flux through the constantan disk. If the backside of the disk is properly insulated, the received heat flows to the edges of the disk and drains out to a copper block which acts as a heat sink. This produces a temperature difference between the center (A or A ) and edges (B or B ) of the disk. The heat flux is obtained by calibrating the temperature difference.
2. Device and fabrication Fig. 3 shows the principle of the bulk-micromachined micro heat-flux sensor that measures heat flux from the wall. The heat flux from the wall is transferred to the sensor through the contact surface and goes out to the environment along the thermal path, A → B → C → D → E, producing a temperature difference along its path. The heat flux from the wall can be obtained by measuring and calibrating the temperature difference between points C and D. Fig. 4 shows the crosssection and top view of the sensor. Thermal path layers of copper (Cu) electroplated on both sides of the wafer are connected through a bulk-micromachined window. To enhance sensor sensitivity in conditions of low heat flux, the thermal path is placed in the vertical direction (perpendicular to the thermal resistance layer, A → B → C in Fig. 3) as well as in the lateral direction (along the thermal resistance layer, C → D in Fig. 3). This change of design enables extension of
Fig. 2. Principle of the circular foil type heat-flux sensor in (a) radiative and (b) conductive heat-flux situations.
the distance between two thermocouple junctions C and D , which effectively increases the thickness of the thermal resistance layer. Fig. 5 shows the principle of the petal shape of the thermopile. The temperatures at points 1, 2, . . . N are the same and so are the temperatures at points 1 , 2 , . . . N . The output signal corresponding to the temperature difference between points 1 and 1 (or 2 and 2 , . . ., or N and N ) is amplified N times using the thermopile of 2N thermocouple junctions.
Fig. 3. Principle of the bulk-micromachined micro heat-flux sensor.
S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
Fig. 4. Schematic view of: (a) cross-section and (b) top view of the micro heat-flux sensor.
Fig. 6 shows the fabrication process of the sensor. The fabrication starts with a bare 3 in. 100-oriented 400 m thick silicon wafer. A 0.6 m thick thermal silicon oxide layer is grown in wet atmospheric conditions. After backside patterning and oxide etching, bulk-micromachining in 25 wt% TMAH solution at 90 ◦ C for 7 h and 10:1 BHF solution in 1 min is used to form a opening window of dimension
Fig. 5. Principle of the petal shape of the thermopile.
Fig. 6. Fabrication process for the heat-flux sensor.
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S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
micromachined window and the thermocouple junctions. The wrinkled membrane of the window is caused by residual stress in the electroplated Cu layer.
3. Test and calibration
Fig. 7. Pictures of the fabricated micro heat-flux sensor: (a) completed heatflux sensor and (b) opening window and the thermocouple junctions.
250 m × 250 m on the front side. Another 1 m thick layer of thermal silicon oxide is grown to be a thermal insulation layer for preventing heat flow through the silicon substrate. After thermal evaporation of 0.01 m thick layer of Cr and 0.1 m thick layer of Au on both sides of the wafer, 10 m thick layer of Cu is electroplated on both sides of the wafer to form thermal paths from the backside to the front side of the wafer. The Cr and Au layers function as an adhesion layer for Au evaporation and a seed layer for Cu electroplating, respectively. A 0.2 m thick electric insulation layer of spinon-glass (SOG) is spin-coated and cured. Two 0.05 m thick thermocouple metals, Ni and Cr, are thermally evaporated and patterned to create the petal-shaped thermopile using a lift-off process. Finally, another 0.2 m thick insulation layer of SOG is spin-coated, cured and patterned to provide wire bonding electrodes and thermal paths to the environment. Fig. 7(a) shows pictures of the completed heat-flux sensor. Fig. 7(b) shows an expanded picture of the bulk-
In this study, we chose a calibration method using a convection facility since heat transfer in a convective environment is the most frequently encountered situation. Fig. 8 shows a schematic view of the experimental setup of the small wind tunnel. In a convective environment, the most important parameter is the heat transfer coefficient at the surface where heat transfer occurs. Even though the same heat flux occurs, the output signal of the sensor may change with the heat transfer coefficient because of the change in the thermal resistance. Accounting for the effect of the heat transfer coefficient is essential in measuring heat flux in a convective environment. In the case of forced convection, the Nusselt number, c Nu = hL k , a non-dimensional form of the heat transfer coefc ficient, is a function of the Reynolds number, Re = ρVL μ and the Prandtl number, Pr = αν , where h is the heat transfer coefficient, Lc the characteristic length, k the fluid conductivity, ν the kinematic viscosity, α the fluid thermal diffusivity, ρ the fluid density, V the free stream velocity and μ is the dynamic viscosity. Since the Prandtl number is regarded as constant in air, the Nusselt number can be represented as a function of the Reynolds number. Thus, in our tests, the heat-flux sensor is calibrated with the Reynolds number. The design of the wind tunnel used to produce the convective environment is adopted from Holemberg et al. [9]. Their report documents the National Institute of Standards and Technology (NIST) standard convective facility in which heat-flux sensors are calibrated. The flow of the wind tunnel is generated by a blower with an ac motor and a variable frequency driver. It flows through a flexible hose, a first diffuser, a heat exchanger, a second diffuser, a nozzle, a settling chamber, a contraction chamber and a test section, as shown in Fig. 8.
Fig. 8. Schematic view of the small wind tunnel.
S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
The flexible hose isolates the wind tunnel from blower vibration. The first diffuser is 899 mm long unit and expands the flow to the final section which has dimensions 260 mm × 260 mm. The heat exchanger is an aluminum, liquid-to-air and fin-tube type. A constant temperature bath is connected to a heat exchanger to maintain the bulk flow at constant temperature. The second diffuser is a 200 mm long unit and expands flow to a final cross-section of 300 mm × 300 mm. The shape of the aluminum nozzle is adopted from the American Society of Mechanical Engineers (ASME) standards for orifices and nozzles. By measuring the difference in pressure before and after the nozzle, the volumetric flow rate is evaluated. The flow from the nozzle goes into the 1200 mm long settling chamber. A screen is inserted between the settling chamber and the contraction chamber. To reduce turbulence intensity, a 2-D contraction of contraction ratio 30:1 is used. The design of the curved end is based on the results of Morel [10]. After the contraction chamber, the flow finally arrives in the test section where the heat-flux sensor is placed. Fig. 9 shows pictures of the test section. The test section is a 20 mm thick copper block under which a heater is placed. The sensor is placed on this copper block, which is used to transform the non-uniform heat flux from the heater into a uniform heat flux over the sensing surface. Thermal insulation is used to reduce backward heat loss.
Fig. 9. Pictures of the test section: (a) sensor on the copper block of the test section and (b) outside of the test section.
585
Fig. 10. Result of the calibration of the heat-flux sensor with the Reynolds number, Re.
4. Results and discussions Fig. 10 shows the calibration result from measurements of the output voltage of the heat-flux sensor with changing heat flux at various values of the Reynolds number. For a given Reynolds number, the output voltage of the sensor increases as the heat flux increases. For a given heat flux, the output voltage is fairly constant for Reynolds numbers of 4000–8000 and decreases after Reynolds number of 8000, except for the high heat flux condition (HF = 180 mW/cm2 ), under which the output voltage decreases as the Reynolds number increases in the range of Reynolds number of 4000–12,000. The latter phenomenon seems to be due to the difference in thermal resistance between the part of the surface of the copper block where the sensor is placed and the part of the surface with no sensor. Since the sensor itself provides thermal resistance in heat transfer situations, heat-flux lines are distorted near the sensor. At higher Reynolds numbers, the heat transfer coefficient increases and the thermal resistance on the surface of the copper block decreases, without changing the thermal resistance of the sensor. This effect causes a relatively lower amount of heat flux to drain out to the environment through the sensor and, therefore, decreases the output voltage of the sensor. When the Reynolds number is 0, the output voltage is quite low, since it is difficult for the thermal energy in the copper block to drain out to the environment. The calibrated sensitivity of the heat-flux sensor is in the range 0.17–1.90 V/(mW cm−2 ) in the heat flux range 0–180 mW/cm2 . These values are about 20 times higher than those from Hager’s heat-flux sensor [5]. However, our sensor does not display good linearity.
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S.H. Oh et al. / Sensors and Actuators A 132 (2006) 581–586
a series of thermocouple, of electroplated Ni–Cr has the petal shape to amplify the output signal of the sensor. For the calibration of the sensor, a small wind tunnel was designed and fabricated under the NIST standard for a convective environment. The calibrated sensitivity of the sensor is 0.17–1.90 V/(mW cm−2 ) in the heat flux range 0–180 mW/cm2 . For a given Reynolds number, the output voltage increases as the heat flux from the wall increases. For a given heat flux, the output voltage is fairly constant for Reynolds numbers of 4000–8000, except for the high heat flux condition HF = 180, under which the output voltage decreases as the Reynolds number increases.
Acknowledgement This work was supported by the Brain Korea 21 Project.
Fig. 11. Effect of the Reynolds number on the output voltage of the heat-flux sensor.
Fig. 11 shows the effect of the Reynolds number on the output voltage of the heat-flux sensor. For a given heat flux condition, the output voltage is fairly constant for Reynolds numbers of 4000–8000, except for the high heat flux condition HF = 180, under which the output voltage decreases with increasing Reynolds. The decrease of output voltage at high Reynolds number and the high heat flux conditions seems to be due to the thermal resistance of the heat-flux sensor. The higher the Reynolds number and the heat flux, the bigger the effect of the thermal resistance of the heat-flux sensor.
5. Conclusions A micro heat-flux sensor with high sensitivity under conditions of low heat flux has been designed, bulk-micromachined and tested in a convective environment. The sensor is based on the circular foil type heat-flux gauge, which is composed of thermal paths and a thermopile. Thermal paths of electroplated Cu on the both sides of a silicon wafer are connected through a bulk-micromachined window and the thermopile,
References [1] T.E. Diller, Advances in heat flux measurements, Adv. Heat Transfer 23 (1983) 279–368. [2] M. Hayashi, A. Sakrai, S. Aso, A study of a multi-layered thin film heat transfer gauge and a new method of measuring heat transfer rate with it, Jpn. Soc. Aeronaut. Space Sci. Trans. 30 (1987) 91– 101. [3] A.H. Epstein, G.R. Guenette, R.J.G. Norton, Y. Gao, High-frequency response heat-flux gauge, Rev. Sci. Instrum. 57 (1985) 639–649. [4] J.M. Hager, S. Simmons, D. Smith, S. Onishi, L.W. Langley, T.E. Diller, Experimental performance of a heat flux microsensor, ASME J. Eng. Gas Turbines Power 113 (1991) 246–250. [5] J.M. Hager, S. Onishi, L.W. Langley, T.E. Diller, Heat flux microsensors, in: R.K. Shah (Ed.), Heat Transfer Measurements, Analysis and Flow Visualization, ASME, New York, 1989, pp. 1–8. [6] R. Gordon, An instrument for the direct measurement of intense thermal radiation, Rev. Sci. Instrum. 24 (5) (1953) 366–370. [7] R. Gordon, A transducer for the measurement of heat flow rate, J. Heat Transfer l82 (1960) 396–398. [8] C.H. Kuo, A.K. Kulkarni, Analysis of heat flux measurement by circular foil gauges in a mixed convection/radiation environment, J. Heat Transfer 113 (1991) 1037–1040. [9] D. Holemberg, K. Steckler, C. Womeldorf, W. Grosshandler, Facility for calibrating heat flux sensors in a convective environment, Proc. ASME Heat Transfer Div. 3 (1997) 165–171. [10] T. Morel, Design of two-dimensional wind tunnel contractions, J. Fluid Eng. (1997) 371–377.