Engineering Materials For Efficient Heat Dissipation In Aerospace
What Is Heat Dissipation?
Transfer of energy from a more concentrated form (Hotter) to less concentrated form (Colder) simply is heat dissipation. It is an irreversible process and its always associated with an increase in entropy thus making this process entropically favorable.
Importance Of Efficient Heat Dissipation Systems
- For maintaining optimum working temperature
- To improves efficiency of the system
- To Increases the working life of the equipment
- And above all its required to ensure Safety
Need For Thermal Protection In Aerospace
The space vehicles encounter high heat flux and temperature during re-entry into the atmosphere generally in the order of 2500°C.
Therefore special Thermal Protection Systems (TPS) is employed to protect the rocket hardware and re-entry capsule from such extreme thermal environments. The Thermal Production System protects the body from severe heating encountered during hypersonic flights through the planetary atmosphere.
Ablative Composites for Thermal Protection in Aerospace
What is Ablation
Ablation is a heat and mass transfer process in which a large amount of heat is dissipated in a very short period of time with sacrificial loss of material. Deep down its a complex process with many physical and chemical transformation. (vaporization, sublimation, pyrolysis etc).
Why Ablative Composites
As we know that we are dealing with temperatures around 2500°C at this point its very difficult to get any normal material whose working temperature range falls into this category. Thus, High temperature resistant metals or alloys alone cannot survive such operating conditions. The metallic structure provides the necessary structural capability but high performance composite materials are required for thermal protection. Ablative composites generally use Carbon or Silica as the reinforcement and phenolic resin as the matrix.
Ablative Mechanism
When an ablative composite is subjected to a very extreme thermal conditions such that the surface temperature increases rapidly (exactly as the situation encountered during the re entry of the space capsule) the temperature starts building up on the surface of the composite, since the resin employed for these kind of composites have very low thermal conductivity the heat transfer to the backup material or the Aluminum core of the capsule is very slow and minimal.
Now as the temperature increases with time and reaches pyrolysis temperature of the resin the decomposition of the composite takes place, Pyrolysis is nothing but the thermal decomposition of the material at elevated temperatures, this leads to char formation over the surface of the composite (char is residue rich in carbon). And as the time pass the region of char advance and the surface recession of the Ablative Composite takes place, as one layer erodes the next layer gets exposed.
The above diagram represents the cross section of the ablative composite over the space capsule, the top layer is the char layer which is exposed directly to the environment. This layer gradually gets eroded as time passes due to harsh conditions. And the second layer is the pyrolysis zone where the decomposition of the composite takes place and the third layer is the composite which is yet to be decomposed and the final fourth layer is the back up material or the core of the capsule which is to be protected.
The pyrolysis gases produced as a result of thermal decomposition created a high pressure region in the Composite / Char interface this creates an outward flow of the gases through the porous char. In this process the gas also takes the heat from the char region to the external region thus further reducing the heat conduction to the successive composite layer thus providing some more resistive time for the capsule.
So in a short note we are designing material to burn away and dissipate heat. So while designing the thermal insulation we are supposed to know the exact rate of at which it burns to determine the optimum thickness of the thermal protection. As its highly important to have minimum payload of the space craft at the same time we must also ensure its safety.
Something Interesting
The above picture is the representation of the heat shield used in Galileo space probe before and after hitting the Jupiter’s atmosphere. We can observe that there is significant change in its surface, this due the decomposition of the ablative composite.
Ablative composites is really a fascinating topic which gained lots of attention in the late 20th century especially in the field of aerospace due to its highly efficient heat shielding properties. Further many studies and research are being carried out in this field to come up with various combination of materials in order to produce an highly efficient composite.
I hope this glimpse through the topic of Ablative composite was informative and interesting. Various simulation are employed to illustrate its real life application we will look into it in upcoming weeks.
