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GB/T 3139-2005 English PDF (GB/T3139-2005)
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GB/T 3139-2005: Fiber-reinforced plastics composites -- Determination of thermal conductivity
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GB/T 3139-2005
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 83.120
Q 23
Replacing GB/T 3139-1982
Fiber-reinforced plastics composites - Determination
of thermal conductivity
ISSUED ON: MAY 18, 2005
IMPLEMENTED ON: DECEMBER 01, 2005
Issued by: General Administration of Quality Supervision, Inspection and
Quarantine;
Standardization Administration of PRC.
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Normative references ... 4
3 Terms and definitions ... 4
4 Test principle ... 5
5 Specimen ... 5
6 Test equipment ... 6
7 Test conditions ... 7
8 Test procedures ... 7
9 Calculation ... 8
10 Test report ... 9
Foreword
This standard replaces GB/T 3139-1982 "Test method for thermal conductivity
of glass fiber reinforced plastics".
Compared with GB/T 3139-1982, the main changes of this standard are as
follows:
- CHANGE the title from "Test method for thermal conductivity of glass fiber
reinforced plastics" to "Fiber-reinforced plastics composites - Determination
of thermal conductivity";
- ADD a chapter of normative references (see Chapter 2);
- ADD a chapter of terms and definitions (see Chapter 3);
- ADD a chapter of test principles (see Chapter 4);
- CHANGE the unit of thermal conductivity coefficient from calories/cm • sec
• °C to Watt per meter [Kelvin] [W/ (m • K)];
- MODIFY the sample pretreatment conditions (1.4 of the 1982 edition; 8.2
of this edition);
- ADD a schematic diagram of the test equipment (see Figure 1).
This standard was proposed by China Building Materials Industry Association.
This standard shall be under the jurisdiction of the National Fiber Reinforced
Plastics Standardization Technical Committee.
Drafting organization of this standard: Beijing FRP Research and Design
Institute.
The main drafters of this standard: Zhao Guangfu, Zhang Liping, Ji Guodong.
This standard was first published in July 1982 and revised for the first time in
May 2005.
Fiber-reinforced plastics composites - Determination
of thermal conductivity
1 Scope
This standard specifies the test principles, specimens, test instruments, test
conditions, test procedures, calculation results and test reports for determining
the thermal conductivity of fiber-reinforced plastics by the guard hot plate
method.
This standard applies to the determination of the thermal conductivity of fiber-
reinforced plastics.
2 Normative references
The provisions in following documents become the provisions of this Standard
through reference in this Standard. For the dated references, the subsequent
amendments (excluding corrections) or revisions do not apply to this Standard;
however, parties who reach an agreement based on this Standard are
encouraged to study if the latest versions of these documents are applicable.
For undated references, the latest edition of the referenced document applies.
GB/T 1446-2005 Fiber-reinforced plastics composites - The generals
3 Terms and definitions
The following terms and definitions apply to this standard.
3.1
Heat flow rate
The heat passing through a surface per unit time.
[GB/T 3102.4-1993, definition 4-7]
3.2
Density of heat flow rate
The heat flow rate per unit area perpendicular to the direction of heat flow.
[GB/T 4132-1996, definition 6.2.3]
3.3
Thermal conductivity
A physical indicator of the thermal conductivity of a material. The value is
equal to the density of heat flow rate divided by the negative temperature
gradient.
[GB/T 4132-1996, definition 6.2.5]
3.4
Mean temperature
In the steady state, the arithmetic average of the high-temperature surface’s
temperature and low-temperature surface’s temperature of the specimen; it
can also be referred to as the average temperature for short.
3.5
Temperature difference
In the steady state, the difference between the high-temperature surface’s
temperature and the low-temperature surface’s temperature of the specimen.
4 Test principle
The guard hot plate method is in a steady state to make a one-way heat flow
flows vertically through a plate-shaped specimen; by measuring the one-
dimensional constant heat flow in the specified heat transfer area and the
temperature difference between the cold and hot surfaces of the specimen, it
can calculate the thermal conductivity of the specimen.
5 Specimen
5.1 The specimen preparation shall be in accordance with the provisions of
Chapter 4 of GB/T 1446-2005.
5.2 The specimen size shall meet the following requirements:
5.2.1 The side length or diameter of the specimen shall be equal to the heating
plate, usually 100 mm.
5.2.2 The thickness of the specimen is at least 5 mm; the maximum is not more
than 1/10 of its side length or diameter.
5.3 The surface of the specimen shall be smooth; the surface unevenness shall
not be greater than 0.50 mm/m; the two sides of the specimens shall be parallel.
5.4 Each group of specimens shall not be less than 3 pieces.
6 Test equipment
The test equipment is as shown in Figure 1.
1 - Cooling water jacket; 2 - Cold plate; 3 - Specimen; 4 - Main heating plate; 5 -
Thermal insulation materials; 6 - Guard heating plate; 7 - Thermal insulation materials;
8 - Bottom heating plate.
Figure 1 -- Schematic diagram of test equipment
6.1 Heating plate
The heating plate is composed of a main heating plate, a guard heating plate
with a certain gap surrounding the main heating plate, a bottom heating plate.
Each heating plate has an independent heater and a surface plate.
6.1.1 The side length or diameter of the heating plate is generally 100 mm; the
width of the guard heating plate is 1/4 of the side length or diameter of the
heating plate, with proper insulation measures. If there are special measures to
ensure that the measurement error of the thermal conductivity coefficient is
within 8%, this value can also be changed appropriately.
6.1.2 The temperature difference at each point on the surface of the heating
plate is not more than 2% of the temperature difference between the two sides
of the specimen under the steady state, meanwhile the maximum shall not be
Water
inlet Water outlet
more than 0.5 °C. The temperature difference at each point on the surface of
the guard heating plate shall not be more than 5% of the temperature difference
between the two sides of the specimen in the steady state; meanwhile the
maximum shall not be more than 1 °C.
6.1.3 The unevenness of the heating plate surface shall not be greater than
0.25 mm/m.
6.2 Cooling plate
The cooling plate shall have an internal threaded two-way liquid loop; the size
of the cooling plate and the surface state of the contact specimen are the same
as the heating plate, but the cooling plate is not spaced.
6.3 Temperature and power measurement accuracy
The temperature measurement shall be accurate to 1% of the temperature
difference between the two sides of the specimen in the steady state; but the
maximum shall not be greater than 0.5 °C.
Measure the power of the main heating plate, which shall be accurate to 1%.
7 Test conditions
7.1 Test environment
The test environmental conditions shall comply with the provisions of Chapter
3 of GB/T 1446-2005.
7.2 Specimen temperature and temperature difference
The temperature of the heating plate generally does not exceed 260 °C; the
temperature of the cold plate rises from room temperature to the required
temperature. The temperature difference between the two sides of the
specimen is not less than 10 °C.
8 Test procedures
8.1 Visual inspection of specimen
The appearance inspection of the specimen is in accordance with the
provisions of 4.2 in GB/T 1446-2005; the unevenness of the specimen surface
is inspected with a knife-shaped flat ruler.
8.2 Conditioning of specimen state
The state of the specimen is conditioned according to the provisions of 4.4 in
GB/T 1446-2005, or according to the technical requirements of the product.
8.3 Measurement of specimen thickness
Measure the thickness of the specimen at least 4 times, accurate to 0.01 mm;
take the arithmetic average.
8.4 Specimen installation
When installing the specimen, pay attention to eliminate the air interlayer and
apply a certain pressure to the specimen.
8.5 Conditioning balance
Adjust the temperature difference between the main heating plate and the guard
heating plate and between the main heating plate and the bottom heating plate
to achieve a balance. The test error of thermal conductivity coefficient as
caused by the imbalance shall not exceed 1%.
8.6 Measurement
After reaching a steady state, measure the power of the main heating plate and
the temperature difference between the two sides of the specimen, then the test
can be ended. The so-called steady state means that the fluctuation of surface
temperature of the specimen within 30 min is not more than 1% of the
temperature difference between the two sides of the specimen under the
condition that the power of the main heating plate remains unchanged;
meanwhile the maximum shall not be greater than 1 °C.
9 Calculation
The thermal conductivity coefficient is calculated according to formula (1),
taking 2 significant figures.
Where:
λ - Thermal conductivity coefficient, in watts per meter Kelvin [W/ (m • K)];
Φ - The power when the main heating plate is stable, in watts (W);
d - The thickness of specimen, in meters (m);
A - The calculated area of the main heating plate (for a certain test device,
the value is a fixed value), in square meters (m2);
t1 - The high temperature of the specimen, in degrees Celsius (°C);
t2 -The low temperature of the specimen, in degrees Celsius (°C).
Calculate the thermal conductivity coefficient of each specimen; calculate the
average value of each group of specimens.
10 Test report
According to Chapter 7 of GB/T 1446-2005, the test results only need to provide
the thermal conductivity coefficient of each specimen and the arithmetic mean
value of each group of specimens; indicate the high-temperature surface’s
temperature of the specimen or the average temperature of the specimen.
__________ END __________
GB/T 3139-2005
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 83.120
Q 23
Replacing GB/T 3139-1982
Fiber-reinforced plastics composites - Determination
of thermal conductivity
ISSUED ON: MAY 18, 2005
IMPLEMENTED ON: DECEMBER 01, 2005
Issued by: General Administration of Quality Supervision, Inspection and
Quarantine;
Standardization Administration of PRC.
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Normative references ... 4
3 Terms and definitions ... 4
4 Test principle ... 5
5 Specimen ... 5
6 Test equipment ... 6
7 Test conditions ... 7
8 Test procedures ... 7
9 Calculation ... 8
10 Test report ... 9
Foreword
This standard replaces GB/T 3139-1982 "Test method for thermal conductivity
of glass fiber reinforced plastics".
Compared with GB/T 3139-1982, the main changes of this standard are as
follows:
- CHANGE the title from "Test method for thermal conductivity of glass fiber
reinforced plastics" to "Fiber-reinforced plastics composites - Determination
of thermal conductivity";
- ADD a chapter of normative references (see Chapter 2);
- ADD a chapter of terms and definitions (see Chapter 3);
- ADD a chapter of test principles (see Chapter 4);
- CHANGE the unit of thermal conductivity coefficient from calories/cm • sec
• °C to Watt per meter [Kelvin] [W/ (m • K)];
- MODIFY the sample pretreatment conditions (1.4 of the 1982 edition; 8.2
of this edition);
- ADD a schematic diagram of the test equipment (see Figure 1).
This standard was proposed by China Building Materials Industry Association.
This standard shall be under the jurisdiction of the National Fiber Reinforced
Plastics Standardization Technical Committee.
Drafting organization of this standard: Beijing FRP Research and Design
Institute.
The main drafters of this standard: Zhao Guangfu, Zhang Liping, Ji Guodong.
This standard was first published in July 1982 and revised for the first time in
May 2005.
Fiber-reinforced plastics composites - Determination
of thermal conductivity
1 Scope
This standard specifies the test principles, specimens, test instruments, test
conditions, test procedures, calculation results and test reports for determining
the thermal conductivity of fiber-reinforced plastics by the guard hot plate
method.
This standard applies to the determination of the thermal conductivity of fiber-
reinforced plastics.
2 Normative references
The provisions in following documents become the provisions of this Standard
through reference in this Standard. For the dated references, the subsequent
amendments (excluding corrections) or revisions do not apply to this Standard;
however, parties who reach an agreement based on this Standard are
encouraged to study if the latest versions of these documents are applicable.
For undated references, the latest edition of the referenced document applies.
GB/T 1446-2005 Fiber-reinforced plastics composites - The generals
3 Terms and definitions
The following terms and definitions apply to this standard.
3.1
Heat flow rate
The heat passing through a surface per unit time.
[GB/T 3102.4-1993, definition 4-7]
3.2
Density of heat flow rate
The heat flow rate per unit area perpendicular to the direction of heat flow.
[GB/T 4132-1996, definition 6.2.3]
3.3
Thermal conductivity
A physical indicator of the thermal conductivity of a material. The value is
equal to the density of heat flow rate divided by the negative temperature
gradient.
[GB/T 4132-1996, definition 6.2.5]
3.4
Mean temperature
In the steady state, the arithmetic average of the high-temperature surface’s
temperature and low-temperature surface’s temperatur...
Delivery: 9 seconds. Download (& Email) true-PDF + Invoice.
Get Quotation: Click GB/T 3139-2005 (Self-service in 1-minute)
Historical versions (Master-website): GB/T 3139-2005
Preview True-PDF (Reload/Scroll-down if blank)
GB/T 3139-2005
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 83.120
Q 23
Replacing GB/T 3139-1982
Fiber-reinforced plastics composites - Determination
of thermal conductivity
ISSUED ON: MAY 18, 2005
IMPLEMENTED ON: DECEMBER 01, 2005
Issued by: General Administration of Quality Supervision, Inspection and
Quarantine;
Standardization Administration of PRC.
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Normative references ... 4
3 Terms and definitions ... 4
4 Test principle ... 5
5 Specimen ... 5
6 Test equipment ... 6
7 Test conditions ... 7
8 Test procedures ... 7
9 Calculation ... 8
10 Test report ... 9
Foreword
This standard replaces GB/T 3139-1982 "Test method for thermal conductivity
of glass fiber reinforced plastics".
Compared with GB/T 3139-1982, the main changes of this standard are as
follows:
- CHANGE the title from "Test method for thermal conductivity of glass fiber
reinforced plastics" to "Fiber-reinforced plastics composites - Determination
of thermal conductivity";
- ADD a chapter of normative references (see Chapter 2);
- ADD a chapter of terms and definitions (see Chapter 3);
- ADD a chapter of test principles (see Chapter 4);
- CHANGE the unit of thermal conductivity coefficient from calories/cm • sec
• °C to Watt per meter [Kelvin] [W/ (m • K)];
- MODIFY the sample pretreatment conditions (1.4 of the 1982 edition; 8.2
of this edition);
- ADD a schematic diagram of the test equipment (see Figure 1).
This standard was proposed by China Building Materials Industry Association.
This standard shall be under the jurisdiction of the National Fiber Reinforced
Plastics Standardization Technical Committee.
Drafting organization of this standard: Beijing FRP Research and Design
Institute.
The main drafters of this standard: Zhao Guangfu, Zhang Liping, Ji Guodong.
This standard was first published in July 1982 and revised for the first time in
May 2005.
Fiber-reinforced plastics composites - Determination
of thermal conductivity
1 Scope
This standard specifies the test principles, specimens, test instruments, test
conditions, test procedures, calculation results and test reports for determining
the thermal conductivity of fiber-reinforced plastics by the guard hot plate
method.
This standard applies to the determination of the thermal conductivity of fiber-
reinforced plastics.
2 Normative references
The provisions in following documents become the provisions of this Standard
through reference in this Standard. For the dated references, the subsequent
amendments (excluding corrections) or revisions do not apply to this Standard;
however, parties who reach an agreement based on this Standard are
encouraged to study if the latest versions of these documents are applicable.
For undated references, the latest edition of the referenced document applies.
GB/T 1446-2005 Fiber-reinforced plastics composites - The generals
3 Terms and definitions
The following terms and definitions apply to this standard.
3.1
Heat flow rate
The heat passing through a surface per unit time.
[GB/T 3102.4-1993, definition 4-7]
3.2
Density of heat flow rate
The heat flow rate per unit area perpendicular to the direction of heat flow.
[GB/T 4132-1996, definition 6.2.3]
3.3
Thermal conductivity
A physical indicator of the thermal conductivity of a material. The value is
equal to the density of heat flow rate divided by the negative temperature
gradient.
[GB/T 4132-1996, definition 6.2.5]
3.4
Mean temperature
In the steady state, the arithmetic average of the high-temperature surface’s
temperature and low-temperature surface’s temperature of the specimen; it
can also be referred to as the average temperature for short.
3.5
Temperature difference
In the steady state, the difference between the high-temperature surface’s
temperature and the low-temperature surface’s temperature of the specimen.
4 Test principle
The guard hot plate method is in a steady state to make a one-way heat flow
flows vertically through a plate-shaped specimen; by measuring the one-
dimensional constant heat flow in the specified heat transfer area and the
temperature difference between the cold and hot surfaces of the specimen, it
can calculate the thermal conductivity of the specimen.
5 Specimen
5.1 The specimen preparation shall be in accordance with the provisions of
Chapter 4 of GB/T 1446-2005.
5.2 The specimen size shall meet the following requirements:
5.2.1 The side length or diameter of the specimen shall be equal to the heating
plate, usually 100 mm.
5.2.2 The thickness of the specimen is at least 5 mm; the maximum is not more
than 1/10 of its side length or diameter.
5.3 The surface of the specimen shall be smooth; the surface unevenness shall
not be greater than 0.50 mm/m; the two sides of the specimens shall be parallel.
5.4 Each group of specimens shall not be less than 3 pieces.
6 Test equipment
The test equipment is as shown in Figure 1.
1 - Cooling water jacket; 2 - Cold plate; 3 - Specimen; 4 - Main heating plate; 5 -
Thermal insulation materials; 6 - Guard heating plate; 7 - Thermal insulation materials;
8 - Bottom heating plate.
Figure 1 -- Schematic diagram of test equipment
6.1 Heating plate
The heating plate is composed of a main heating plate, a guard heating plate
with a certain gap surrounding the main heating plate, a bottom heating plate.
Each heating plate has an independent heater and a surface plate.
6.1.1 The side length or diameter of the heating plate is generally 100 mm; the
width of the guard heating plate is 1/4 of the side length or diameter of the
heating plate, with proper insulation measures. If there are special measures to
ensure that the measurement error of the thermal conductivity coefficient is
within 8%, this value can also be changed appropriately.
6.1.2 The temperature difference at each point on the surface of the heating
plate is not more than 2% of the temperature difference between the two sides
of the specimen under the steady state, meanwhile the maximum shall not be
Water
inlet Water outlet
more than 0.5 °C. The temperature difference at each point on the surface of
the guard heating plate shall not be more than 5% of the temperature difference
between the two sides of the specimen in the steady state; meanwhile the
maximum shall not be more than 1 °C.
6.1.3 The unevenness of the heating plate surface shall not be greater than
0.25 mm/m.
6.2 Cooling plate
The cooling plate shall have an internal threaded two-way liquid loop; the size
of the cooling plate and the surface state of the contact specimen are the same
as the heating plate, but the cooling plate is not spaced.
6.3 Temperature and power measurement accuracy
The temperature measurement shall be accurate to 1% of the temperature
difference between the two sides of the specimen in the steady state; but the
maximum shall not be greater than 0.5 °C.
Measure the power of the main heating plate, which shall be accurate to 1%.
7 Test conditions
7.1 Test environment
The test environmental conditions shall comply with the provisions of Chapter
3 of GB/T 1446-2005.
7.2 Specimen temperature and temperature difference
The temperature of the heating plate generally does not exceed 260 °C; the
temperature of the cold plate rises from room temperature to the required
temperature. The temperature difference between the two sides of the
specimen is not less than 10 °C.
8 Test procedures
8.1 Visual inspection of specimen
The appearance inspection of the specimen is in accordance with the
provisions of 4.2 in GB/T 1446-2005; the unevenness of the specimen surface
is inspected with a knife-shaped flat ruler.
8.2 Conditioning of specimen state
The state of the specimen is conditioned according to the provisions of 4.4 in
GB/T 1446-2005, or according to the technical requirements of the product.
8.3 Measurement of specimen thickness
Measure the thickness of the specimen at least 4 times, accurate to 0.01 mm;
take the arithmetic average.
8.4 Specimen installation
When installing the specimen, pay attention to eliminate the air interlayer and
apply a certain pressure to the specimen.
8.5 Conditioning balance
Adjust the temperature difference between the main heating plate and the guard
heating plate and between the main heating plate and the bottom heating plate
to achieve a balance. The test error of thermal conductivity coefficient as
caused by the imbalance shall not exceed 1%.
8.6 Measurement
After reaching a steady state, measure the power of the main heating plate and
the temperature difference between the two sides of the specimen, then the test
can be ended. The so-called steady state means that the fluctuation of surface
temperature of the specimen within 30 min is not more than 1% of the
temperature difference between the two sides of the specimen under the
condition that the power of the main heating plate remains unchanged;
meanwhile the maximum shall not be greater than 1 °C.
9 Calculation
The thermal conductivity coefficient is calculated according to formula (1),
taking 2 significant figures.
Where:
λ - Thermal conductivity coefficient, in watts per meter Kelvin [W/ (m • K)];
Φ - The power when the main heating plate is stable, in watts (W);
d - The thickness of specimen, in meters (m);
A - The calculated area of the main heating plate (for a certain test device,
the value is a fixed value), in square meters (m2);
t1 - The high temperature of the specimen, in degrees Celsius (°C);
t2 -The low temperature of the specimen, in degrees Celsius (°C).
Calculate the thermal conductivity coefficient of each specimen; calculate the
average value of each group of specimens.
10 Test report
According to Chapter 7 of GB/T 1446-2005, the test results only need to provide
the thermal conductivity coefficient of each specimen and the arithmetic mean
value of each group of specimens; indicate the high-temperature surface’s
temperature of the specimen or the average temperature of the specimen.
__________ END __________
GB/T 3139-2005
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 83.120
Q 23
Replacing GB/T 3139-1982
Fiber-reinforced plastics composites - Determination
of thermal conductivity
ISSUED ON: MAY 18, 2005
IMPLEMENTED ON: DECEMBER 01, 2005
Issued by: General Administration of Quality Supervision, Inspection and
Quarantine;
Standardization Administration of PRC.
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Normative references ... 4
3 Terms and definitions ... 4
4 Test principle ... 5
5 Specimen ... 5
6 Test equipment ... 6
7 Test conditions ... 7
8 Test procedures ... 7
9 Calculation ... 8
10 Test report ... 9
Foreword
This standard replaces GB/T 3139-1982 "Test method for thermal conductivity
of glass fiber reinforced plastics".
Compared with GB/T 3139-1982, the main changes of this standard are as
follows:
- CHANGE the title from "Test method for thermal conductivity of glass fiber
reinforced plastics" to "Fiber-reinforced plastics composites - Determination
of thermal conductivity";
- ADD a chapter of normative references (see Chapter 2);
- ADD a chapter of terms and definitions (see Chapter 3);
- ADD a chapter of test principles (see Chapter 4);
- CHANGE the unit of thermal conductivity coefficient from calories/cm • sec
• °C to Watt per meter [Kelvin] [W/ (m • K)];
- MODIFY the sample pretreatment conditions (1.4 of the 1982 edition; 8.2
of this edition);
- ADD a schematic diagram of the test equipment (see Figure 1).
This standard was proposed by China Building Materials Industry Association.
This standard shall be under the jurisdiction of the National Fiber Reinforced
Plastics Standardization Technical Committee.
Drafting organization of this standard: Beijing FRP Research and Design
Institute.
The main drafters of this standard: Zhao Guangfu, Zhang Liping, Ji Guodong.
This standard was first published in July 1982 and revised for the first time in
May 2005.
Fiber-reinforced plastics composites - Determination
of thermal conductivity
1 Scope
This standard specifies the test principles, specimens, test instruments, test
conditions, test procedures, calculation results and test reports for determining
the thermal conductivity of fiber-reinforced plastics by the guard hot plate
method.
This standard applies to the determination of the thermal conductivity of fiber-
reinforced plastics.
2 Normative references
The provisions in following documents become the provisions of this Standard
through reference in this Standard. For the dated references, the subsequent
amendments (excluding corrections) or revisions do not apply to this Standard;
however, parties who reach an agreement based on this Standard are
encouraged to study if the latest versions of these documents are applicable.
For undated references, the latest edition of the referenced document applies.
GB/T 1446-2005 Fiber-reinforced plastics composites - The generals
3 Terms and definitions
The following terms and definitions apply to this standard.
3.1
Heat flow rate
The heat passing through a surface per unit time.
[GB/T 3102.4-1993, definition 4-7]
3.2
Density of heat flow rate
The heat flow rate per unit area perpendicular to the direction of heat flow.
[GB/T 4132-1996, definition 6.2.3]
3.3
Thermal conductivity
A physical indicator of the thermal conductivity of a material. The value is
equal to the density of heat flow rate divided by the negative temperature
gradient.
[GB/T 4132-1996, definition 6.2.5]
3.4
Mean temperature
In the steady state, the arithmetic average of the high-temperature surface’s
temperature and low-temperature surface’s temperatur...
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