  Luminous red novae (LRNe) and their connection to common envelope evolution
(CEE) remain elusive in astrophysics. Here, we present a radiation hydrodynamic
model capable of simulating the light curves of material ejected during a CEE.
For the first time, the radiation hydrodynamic model incorporates complete
recombination physics for hydrogen and helium. The radiation hydrodynamic
equations are solved with Guangqi. With time-independent ejecta simulations, we
show that the peaks in the light curves are attributed to radiation-dominated
ejecta, while the extended plateaus are produced by matter-dominated ejecta. To
showcase our model's capability, we fit the light curve of AT2019zhd. The
central mass object of $6M_{\odot}$ is assumed based on observations and
scaling relations. Our model demonstrates that the ejecta mass of AT2019zhd
falls within the range of $0.04M_{\odot}$ to $0.1M_{\odot}$. Additionally, we
demonstrate that recombination energy and radiation force acceleration
significantly impact the light curves, whereas dust formation has a limited
effect during the peak and plateau phases.
