YouDao has an example of 我去洗把脸:
Zhīdàole, mā. Wǒ qù xǐ bǎ liǎn, ràng zìjǐ qīngxǐng yīxiē.
I see, Mummy. I'm going to wash my face and wake myself up.
There's similar examples at YouDao for 洗把脸.
Question: Why is 把 in 我去洗把脸?
I see no real reason for 把 to be in there. My guess is that it's a variation on 把脸洗.