diff --git a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/combustion ExamplesWeek23-updated26-5-21.docx b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/combustion ExamplesWeek23-updated26-5-21.docx new file mode 100644 index 0000000..a08fa0f Binary files /dev/null and b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/combustion ExamplesWeek23-updated26-5-21.docx differ diff --git a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/consolidation_combustion.docx b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/consolidation_combustion.docx new file mode 100644 index 0000000..150164d Binary files /dev/null and b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/consolidation_combustion.docx differ diff --git a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer ConsolidationWeek24.docx b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer ConsolidationWeek24.docx new file mode 100644 index 0000000..c09cc93 Binary files /dev/null and b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer ConsolidationWeek24.docx differ diff --git a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer consolidation-solution.docx b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer consolidation-solution.docx new file mode 100644 index 0000000..548056a Binary files /dev/null and b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer consolidation-solution.docx differ diff --git a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer solutions.pdf b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer solutions.pdf new file mode 100644 index 0000000..6d85685 Binary files /dev/null and b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/exercise_sheets/heat transfer solutions.pdf differ diff --git a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/heat_transfer.md b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/heat_transfer.md index 2bda7e8..4501548 100755 --- a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/heat_transfer.md +++ b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/heat_transfer.md @@ -9,12 +9,24 @@ lecture_notes: [ ./lecture_notes/ConvectHeatTrans2022-2023.pdf ] exercise_sheets: [ ./exercise_sheets/ExamplesConvectionHeatTransfer.pdf ] --- +# Errata + +## Grashof Number (formula booklet, lecture slides p. 24, lecture recording 2, 1:19:30) + +The Grashof number formula should be written with $\nu$, not $\mu$, +It is correctly written in lecture notes (p. 8), [Wikipedia](https://en.wikipedia.org/wiki/Grashof_number), +and on this page. + +Somehow it is also incorrect in the formula booklet. + +You can check which one is correct by checking which results in a dimensionless Grashof number. + # Convection - conduction and radiation heat transfer can be estimated by calculations and properties - convection is dependent on fluid properties, flow type, and flow characteristics -The rate of convective heat transfer, $\dot Q$, is given by +The rate of convective heat transfer, $\dot Q$, is given by Newton's law of cooling: \begin{equation} \dot Q = hA(T_f-T_w) @@ -33,3 +45,91 @@ $$\dot Q = \frac{T_f-T_w}{\sum R_\text{thermal}}$$ where $R_\text{thermal} = \frac{1}{hA}$. In a way this analogous to Ohm's law, specifically with resistors in series ($I = \frac{\Delta V}{\sum R_\text{electrical}}$). + +## Analysis of how Convection Works + +At a hot wall, the velocity of the fluid touching it will be zero. +Therefore the heat transfer into the fluid must happen by conduction. +This means that the local heat flux per unit area, $\dot Q''$ (dot for rate, double dash for per +unit area) is given by: + +$$\dot Q'' = -k \frac{\delta T}{\delta y}|_{\text{wall}}$$ + +where $k$ is the conductivity of the wall + +![](./images/convection.png) + +The heated fluid is carried away by convection, therefore at steady state we can say that: + +$$\dot Q'' = -k \frac{\delta T}{\delta y}|_\text{wall} = -h(T_\infty - T_\text{wall})$$ + +Rearranging allows $h$ to be found. + +# Nusselt Number - Relation Between Fluid Conductivity and Convection + +Nusselt number is a dimensionless number: + +$$\text{Nu} = \frac{hL}{k_f}$$ + +where $k_f$ is conductivity of the fluid, $L$ is the representative length (e.g. diameter, length, +internal width, etc.). + +Since $h$ is unknown a lot of the time, sometimes Nusselt number must be found through approximating +by other dimensionless numbers: Prandtl, Reynolds, and Grashof. + +Nusselt number for a laminar forced flow is around 3.66. +For a turbulent forced flow it is estimated to be: + +$$\text{Nu}_x = 0.023\text{Re}_x^{0.8}\text{Pr}^{0.4}$$ + +For a laminar forced flow over a flat plate: + +$$\text{Nu}_x = 0.332\text{Re}_x^{0.5}\text{Pr}^{0.33}$$ + +For natural convection of a vertical wall: + +\begin{align*} +\text{Nu}_x = 0.59(\text{Gr}_x\text{Pr})^{0.25} &\text{ for }10^3 < \text{GrPr} < 10^9 \\ +\text{Nu}_x = 0.13(\text{Gr}_x\text{Pr})^{0.25} &\text{ for }10^9 < \text{GrPr} < 10^{12} +\end{align*} + +[removing annoying html syntax highlighti]: > + +# Prandtl Number + +This number relates thickness of velocity boundary layer to thickness of thermal boundary layer: + +$$\text{Pr} = \frac{c_p\mu}{k_f} = \frac{\nu}{\alpha}$$ + +where $\nu$ is the kinematic viscosity and $\alpha$ is the thermal diffusivity (equations given in +lecture notes p. 5). + +# Grashof Number + +Grashof number compares the buoyancy of the fluid (due to compressibility, $\beta = T^{-1}$, +where $T$ is the film temperature, or average temperature between fluid and wall, in kelvin) +and the viscous resistance to buoyant motion. + +$$\text{Gr} = \frac{g\beta L^3\rho^2\Delta T}{\nu^2}$$ + +where $g$ is acceleration due to gravity, $L$ is the height or length of the tube, $\rho$ is density +of the fluid, $\Delta T = T_\text{wall} - T_\infty$, and $\nu$ is the kinematic viscosity. + +# Axisymmetric Shenanigans + +Axisymmetric shapes are symmetric about an axis. + +$$\dot Q' = -kA\frac{\mathrm dT}{\mathrm dr} = -k2\pi r\frac{\mathrm dT}{\mathrm dr}$$ + +Note that the single dash on $\dot Q'$ implies *per unit length*. +For any length, $L$, $A = 2\pi rL$. + +In this case, the temperature profile is no longer linear, even if $k$ is constant: + +$$R_\text{th} = \frac{\ln r_o - \ln r_i}{2\pi kL}$$ + +# Definitions + +- *lagging* - insulation + + diff --git a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/images/convection.png b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/images/convection.png new file mode 100644 index 0000000..31a3b5f Binary files /dev/null and b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/images/convection.png differ diff --git a/uni/mmme/2047_thermodynamics_and_fluid_dynamics/lecture_slides/T4 - Dimensional analysis - complete.pdf b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/lecture_slides/T4 - Dimensional analysis - complete.pdf new file mode 100644 index 0000000..cfd4115 Binary files /dev/null and b/uni/mmme/2047_thermodynamics_and_fluid_dynamics/lecture_slides/T4 - Dimensional analysis - complete.pdf differ