@ umarabas
but what is role of spongy material in flask i think its vaccum area and what heat lost through it is by convection.
so acctuly all three process of heat transfer is involved in flask.
am i righ?

the spongy material in the flask is an insulator. so even if some energy is being absorbed by the shinny reflector jar inside the flask, the insulator (spongy material) ensures that little heat is transferred to the surface of the flask and thus to the surroundings.

no convection cannot happen over here. to understand different between the three lemme give you a brief description. i hope you will understand it easily what process could be happening here:

1- Conduction: it's caused by the vibrations of atoms within the material. so with heat supply the atoms vibrates more and more. just like in case of balls, the atoms strike the atom next to them, and so on. thus a whole chain process begins. that is why if we heat one end of a pole, the heat is felt at the other end as well after some time. this is called conduction. No movement of atoms takes place!

2- Convection: Mostly happens in fluids (liquids and gases). When atoms/molecules are heated they gain energy. since they are loosely held together, therefore they are free to move. the more the energy, the more they move around. the vacant places left by the heated atoms are filled by other atoms. thus we see that water gets warm throughout the bucket when you mix hot and cold water. this is convection and it involves movements of atoms/particles/molecules.

3- Radiation: it can happen both in liquids and solids. in simple words, (although the detail is complex) whenever the atoms of any particular substances are heated up, they loose some of the energy to the surroundings (vacuum or air or whatever). how does this happen? its like from a little excited state they return to the non-excited state. therefore the energy they have absorbed is released to the environment. this is called radiation.

so now you see convection is not an agent over here i hope it was of some help!

__________________
Umar Abbas Babar
I Know I am Bad But When Bad is the Best, Bad is the Only Choice

@umarabas
today i read from 1 book name physis written by cutnel in that book thermos botle mechanism is given and in that also said that in thermos bottle all three type of heat transfer is used.
detail as according to book is as under
a themos consist of double walled glas layer with silver wall inside. minimize heat transfer by conduction convection and radiation
the space betwn the wall is evacuated to reduce energy loses due to conduction and convection
the silvered surface reflect most of the radiant energy that would otherwise enter or leave the liquid in the thermossome heat lost becz of rubber like gaskets and stopper etc