$P^2 V=K$

or $P V^{-2}=K$

$N=-2$

$C=C_V+\frac{R}{1-N}=$ positive quantity

$C > 0$

$W > 0 \quad$ [gas is expanding]

$\because P V^{-2}=K$ so $T V^{-3}=$ constant

$\Rightarrow T$ will increases if $V$ increases.

$\Rightarrow \Delta T > 0$

So $\Delta U=\eta C \Delta T > 0$

It will increase continuously.