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Firefighting job requires a special clothing, a protective clothing is typically constructed of three layers separated by air gaps, as shown in the figure below. - Shell (s) Moisture barrier (mb) Thermal liner (tl) Fire-side mm mm- Firefighter k, L. Lob Air gap -Air gap Figure 1. Firefighting suit protective layers. Representative dimensions and thermal conductivities for the layers are as presented in the table below: Table 1: The thickness and thermal conductivity for different layers in a firefighters suit Layer Thickness (mm) k (W/m · K) Shell (s) 0.8 0.047 Moisture barrier (mb) Thermal liner (tl) 0.55 0.012 3.5 0.038 The air gaps between the layers are 1 mm thick, and heat is transferred by conduction and radiation exchange through the stagnant air. The linearized radiation coefficient for a gap may be approximated as hond = 0(T; + T; )(T² +T;’) = 4oTmx', where Tavg represents the average temperature of the surfaces comprising the gap, and the radiation flux across the gap may be expressed as q"mt = h, (T7; – T; ) (a) Discuss how to determine the heat transfer through a composite wall, while defining the thermal resistance for different heat transfer modes. (b) Represent Figure 1 in a thermal circuit, labelling all the thermal resistances. Calculate and tabulate the thermal resistances per unit area for each of the layers, as well as for the conduction and radiation processes in the gaps. Assume that a value of Tavg=470 K may be used to approximate the radiation resistance of both gans Make comment on the relative magnitudes of the resistances

Firefighting job requires a special clothing, a protective clothing is typically constructed of three layers separated by air gaps, as shown in the figure below. - Shell (s) Moisture barrier (mb) Thermal liner (tl) Fire-side mm mm- Firefighter k, L. Lob Air gap -Air gap Figure 1. Firefighting suit protective layers. Representative dimensions and thermal conductivities for the layers are as presented in the table below: Table 1: The thickness and thermal conductivity for different layers in a firefighters suit Layer Thickness (mm) k (W/m · K) Shell (s) 0.8 0.047 Moisture barrier (mb) Thermal liner (tl) 0.55 0.012 3.5 0.038 The air gaps between the layers are 1 mm thick, and heat is transferred by conduction and radiation exchange through the stagnant air. The linearized radiation coefficient for a gap may be approximated as hond = 0(T; + T; )(T² +T;’) = 4oTmx', where Tavg represents the average temperature of the surfaces comprising the gap, and the radiation flux across the gap may be expressed as q"mt = h, (T7; – T; ) (a) Discuss how to determine the heat transfer through a composite wall, while defining the thermal resistance for different heat transfer modes. (b) Represent Figure 1 in a thermal circuit, labelling all the thermal resistances. Calculate and tabulate the thermal resistances per unit area for each of the layers, as well as for the conduction and radiation processes in the gaps. Assume that a value of Tavg=470 K may be used to approximate the radiation resistance of both gans Make comment on the relative magnitudes of the resistances

Elements Of Electromagnetics
Elements Of Electromagnetics
7th Edition
ISBN:9780190698614
Author:Sadiku, Matthew N. O.
Publisher:Sadiku, Matthew N. O.
ChapterMA: Math Assessment
Section: Chapter Questions
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Task 1 Firefighting job requires a special clothing, a protective clothing is typically constructed of three layers separated by air gaps, as shown in the figure below. Moisture barrier (mb) Thermal liner (tl) - Shell (s) Fire-side mm mm Firefighter k, L, Kenb Lmb Air gap Air gap Figure 1. Firefighting suit protective layers. Representative dimensions and thermal conductivities for the layers are as presented in the table below: Table 1: The thickness and thermal conductivity for different layers in a firefighters suit Layer Thickness (mm) k (W/m · K) 0.8 Shell (s) Moisture barrier (mb) Thermal liner (tl) 0.047 0.55 0.012 3.5 0.038 The air gaps between the layers are 1 mm thick, and heat is transferred by conduction and radiation exchange through the stagnant air. The linearized radiation coefficient for a gap may be approximated as had = 0(T, + T,)(T° +T;’) = 40Tm', where Tavg represents the average temperature of the surfaces comprising the gap, and the radiation flux across the gap may be expressed as q",md = h,ma (T; – T; ) (a) Discuss how to determine the heat transfer through a composite wall, while defining the thermal resistance for different heat transfer modes. (b) Represent Figure 1 in a thermal circuit, labelling all the thermal resistances. Calculate and tabulate the thermal resistances per unit area for each of the layers, as well as for the conduction and radiation processes in the gaps. Assume that a value of Tavg= 470 K may be used to approximate the radiation resistance of both gaps. Make comment on the relative magnitudes of the resistances.
Transcribed Image Text:Task 1 Firefighting job requires a special clothing, a protective clothing is typically constructed of three layers separated by air gaps, as shown in the figure below. Moisture barrier (mb) Thermal liner (tl) - Shell (s) Fire-side mm mm Firefighter k, L, Kenb Lmb Air gap Air gap Figure 1. Firefighting suit protective layers. Representative dimensions and thermal conductivities for the layers are as presented in the table below: Table 1: The thickness and thermal conductivity for different layers in a firefighters suit Layer Thickness (mm) k (W/m · K) 0.8 Shell (s) Moisture barrier (mb) Thermal liner (tl) 0.047 0.55 0.012 3.5 0.038 The air gaps between the layers are 1 mm thick, and heat is transferred by conduction and radiation exchange through the stagnant air. The linearized radiation coefficient for a gap may be approximated as had = 0(T, + T,)(T° +T;’) = 40Tm', where Tavg represents the average temperature of the surfaces comprising the gap, and the radiation flux across the gap may be expressed as q",md = h,ma (T; – T; ) (a) Discuss how to determine the heat transfer through a composite wall, while defining the thermal resistance for different heat transfer modes. (b) Represent Figure 1 in a thermal circuit, labelling all the thermal resistances. Calculate and tabulate the thermal resistances per unit area for each of the layers, as well as for the conduction and radiation processes in the gaps. Assume that a value of Tavg= 470 K may be used to approximate the radiation resistance of both gaps. Make comment on the relative magnitudes of the resistances.
(Hint: the thermal conductivity at 470 K equals to 0.0387 W/m.K) (c) For a pre-flash-over fire environment in which firefighters often work, the typical radiant heat flux on the fireside of the turnout coat is 0.25 W/cm?. What is the outer surface temperature of the turnout coat if the inner surface temperature is 66 °C, a condition that would result in burn injury? |(d) Discuss the effect of changing the thermal conductivity (k) of the shell layer in table 1 above from 0.047 to 0.06 W/m.K on the total thermal resistance.
Transcribed Image Text:(Hint: the thermal conductivity at 470 K equals to 0.0387 W/m.K) (c) For a pre-flash-over fire environment in which firefighters often work, the typical radiant heat flux on the fireside of the turnout coat is 0.25 W/cm?. What is the outer surface temperature of the turnout coat if the inner surface temperature is 66 °C, a condition that would result in burn injury? |(d) Discuss the effect of changing the thermal conductivity (k) of the shell layer in table 1 above from 0.047 to 0.06 W/m.K on the total thermal resistance.
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