how can i know what is default Kernel version
By reading the release notes. With every new release, the development team publishes the major changes compared to the previous releases. The kernel version is mentioned under the Key Features section.
The Release Note for 8.1-r4 indicates:
- Update to LTS kernel 4.19.110.
For 8.1-r5:
- Update to LTS kernel 4.19.122.
For 9.0-r2:
Update to LTS kernel 4.19.110.
When the iso file is not for the latest version, then the kernel is also mentioned with the filename.
For example, there are 3 64-bit files for 8.1-r4. The first 2 are for the kernel 4.19.110 (not mentioned on the filename), while the third filename has the kernel mentioned - kernel 4.9.
- 64-bit ISO: android-x86_64-8.1-r4.iso
- 64-bit rpm: android-x86-8.1-r4.x86_64.rpm
- 64-bit ISO with kernel 4.9: android-x86_64-8.1-r4-k49.iso